I just got back from doing laundry and it's 1:20 a.m. and I'm not positive I'm sane right now *grin* -- but the short answer is yes they can 'develop' an allergy to something (or the mfg may have diminished the quality of the ingredients too). But it sounds to me in the op's situation (and I did NOT read the whole thread so forgive me if I'm assuming wrongly here) there have been a couple of food changes pretty quick and after 2 1/2 days she's thinking she's made a mistake.
When I say "food switching" I mean back and forth over pretty short durations between a few different foods to 'find' one they think will be tolerated. When you do that you expose the dog to so many new allergens that you wind up developing MORE allergens because the body is in such a high state of 'alert' allergy-wise that any new thing becomes an irritant.
In honesty, if the focus is food, it's likely something in the environment that is keeping the primary allergies SO stirred up that everything and anything is an allergen.
Sometimes you just plain have to STOP and get the dog calmed back down. And yes, I mean the dreaded "p" word (prednisone) or something (usually a steroid) that will reduce the inflammation and get the skin calmed down. Sometimes an antibiotic is needed too (when you get paws that are red and oozing you've got infection, and probably it's been brewing for a while) because infected skin won't heal.
But you only do that BRIEFLY to get control of the situation and get the skin back to something that approaches normal. Frankly I'd be using antihistamine and anything else I could to get the situation normalized and **then** you start with holistic methods to maintain.
I think most everyone on here who knows me realizes that if there's an herb or an alternative method I'm gonna use it first. BUT ... there comes a time when you have to relieve discomfort and pain and get the situation stablilized first. Herbs often work gently and slowly, so they tend not to be the first line of defense when you have an ugly situation.
And yeah, Billy was on pred a couple of times in my huge drive to get his skin calmed down to STAY that way. I cook for my dogs so altho I didn't do a total elmination diet I was able to change only ONE thing a week so I could determine what might be causing a problem (and I didn't use grain).
With Billy the other seriously important component was altho I had thyroid panels done THREE TIMES, we kept getting a "low but within normal range" answer ... and ultimately I had them send it to Michigan State (they use Dr. Dodd's protocols) and yep, it WAS low. Not severely low but low enough to cause problems.
My vet put him on Armour Thyroid (it's a glandular -- porcine thyroid -- rather than a synthetic thyroid) and within 2 months he was a new dog. Not perfect skin -- no way. But "dealable" -- see thyroid imbalance will simply make anything you ARE doing **not** work, so getting the thyroid balanced simply levels the playing field.
We used a combination of Chinese herbals (one for 'dampness' - which is the papules that appear on the belly and in the moist folks like leg-pits, etc.; and the other for 'external wind' which is the papules that appear on the top of the body pretty much anywhere 'wind' might blow across the body) AND I still gave him 25 mg of diphenhydramine (Benedryl) twice daily -- he's 30 pounds so that was a small but ok dose.
We retested after Christmas and had to adjust the Armour thyroid up a bit more -- and then altho I continued the small dose of Benedryl, I also added some homeopathics 3 times a day which really seemed to help enormously and his skin was great thru the spring.
Now all bets are off because he's being treated for the IMHA so he's in steroid h%ll (and no that doesn't mean the skin is great because at a certain dose it begins having a negative effect)
But atopic allergies (hay fever type allergies and allergies absorbed thru the skin) and food allergies can ping off each other in a big way. The atopic allergies get the body riled up and then the food allergies blow it all out of proportion. Some dogs do have classic food allergies -- like a human might have to peanuts or such a thing. But mostly it's a 'sensitivity' question more than a true allergy question. That's not to say it's imaginary -- just rather than it being cut and dried that the dog is "allergic" to A, B and C -- it's more that when the body is already out of whack and all inflamed from atopic allergies then when food A, B and C might normally be tolerated NOW they are a problem.
See it's like some of the commercially prepared "allergy" diets like Hills Z diet. If you look at the label it's like an allergy nightmare -- all sorts of things that can be allergens. BUT the reason it so often works is because the molecules are broken down so small the body processes it differently and 'ignores' the allergy response.
That's why I'm making the distinction between "allergy" and "sensitivity". If you can bring down the body's response generally, then you can maintain with easier methods. And altho I hate pred with a passion (and remember steroids are all that are keeping my dog healthy right now *sigh*) I also have a healthy respect for what it can do under the right circumstances. You don't use it on an ongoing basis to control 'allergies' nor do you want to get into a situation where you use them 'regularly' (like the periodic "allergy shot" as some vets call steroid shots).
But honestly if this dog has gone to having feet that are THAT sore, then some more radical methods to bring down the allergic response will likely have to be used short term, and then roll over into something more gentle long term.
I hope that makes sense.