This is a bit graphic but will explain in words what your vet can explain by showing you.
It's the glands again and it could also be an infection (particularly by what you're describing back there).
Do her a favor and have the vet *show you* (a woman with smaller hands) HOW to do it internally correctly. Smaller dogs often need their glands expressed more and more frequently - particularly as they mature.
If you are feeding an ultra low bulk, perhaps high protein food (one of those that advertises that your dog will have lovely *small* poops) you may be exacerbating the problem. She may need more fiber -- pumpkin, squash, even grain, to help the poops be *large* enough to sufficiently express the anal glands when she potties.
The glands are inside - if you're looking at her behind like a 'clock' the glands are between 4-5 o'clock and 7-8 o'clock. Think about a piece of bubble wrap -- with the 'bubble' (which is the gland) being on the inside. The hole is on the top of the 'bubble' angled towards the opening.
With KY jelly you lubricate your finger and go below the 'bubble' (one finger in and thumb outside) and you squeeze gently up to express the gland.
I've had small dogs and ones with this problem truly suffer unless you do it frequently and feed to help alleviate the problem. I've had people get all upset thinking a vet *must* do this. But with my dogs who had problems it was a whole lot easier for *them* for me to do it in the tub with lots of time, patience, LOVE and treats and cuddles after. My hands/fingers are smaller than most vets' and thus it's easier on the dog.
We all love hating the "inferior" foods that use a lot of bulk in them -- but this WAS the purpose of the bulk. To make the fecal matter solid enough to press against this gland as it exits the body so it presses against the glad and empties it.
I'm not criticizing what you feed (shoot, I have no clue!) but this is one of those things we aren't told, and for some reason some small dogs have a bigger problem than others.
It's not a fun thing to do for your pet, but it's one I do out of love. It's just another of those distasteful tasks like cleaning up their waste and messing with brushing teeth, etc. that I do because I love them.