Hair loss.... Bumps/pimples appearing.. Scratching and biting

It could be flea allergy dermatitis.

  Hi yousefgreat and welcome to idog; It is most likely not due to hormonal changes; mondayblues may be right; what flea treatment are you using? Also, it could be contact dermititis. If you live in the Southern part of the country there is a plant called Wandering Jew that grows just about everywhere and is a common cause of a rash and itching on the stomach and lower parts of dogs, here's a picture;

When the vet said it could be due to worms; did you mean ringworm? Also, it sounds like it could be mites; has the vet done a skin scraping of the area to check for them?

 Edited to add; If it's a contact allergy applying some Gold Bond Medicated Powder to the area should help.

I'd second everything Jessies_mom said above -- and I'm going to add the possibility of sarcoptic mange to that as well.  It's not likely demodectic mange (that would have started on the face FIRST and usually itching is not that big of a sign), but sarcoptic mange DOES show tiny bumps, etc.
 
A contact allergy would also be my second thot -- the tiny bumps/pustules are typical of a contact allergy and the itch can be fierce.
 
Flea allergy dermatitis -- that's another thot (and it could be more than one thing causing this) -- typically that starts at the base of the tail and moves forward on the back and down the back lets.  But mostly you will see scabbing, not little tiny bumps/pustules.
 
I can't imagine that a vet would say this was due to 'worms' -- MITES yes, but not worms.  (big huge difference)
 
Do you or any of the other humans in the home have any small rashy areas??  Maybe feet/legs, hands/wrists? or even your legs where you may hold or sit next to the dog??  Are YOU scratching (or feel like you are scratching in sympathy??)  If so then that would triple my guess of sarcoptic mange. 
 
Any dog with a lowered immune system can get sarcops -- it's not the result of a dirty house or any other thing -- and frenchies are part of the bully breeds so the immune system does mature more slowly. 
 
If the vet isn't concerned about this then switch vets -- it would worry me that a vet would allow a dog to walk out with out treating such a thing.  And no -- this is NOT likely to be hormonal.  Not at all.
 
 

Hi, I'm new, but I think it is a flea allergy creating the pimples/pustules.  In advanced cases, such as our first dog had, she then popped the pimples and was allergic as well to the fluid in the pimple which lead to large scabs forming, a form of staph infection.  Flea control is going to be key, but also a steriod shot to get the initial problem under control.  In our case I think we also had to add antibiotics.