Diarrhea and scooting bottom on carpet...

Could this be worms, a parasitic infection, or anal glands? Something else?

My poor baby Stella is really going through it. We're still trying to get her passed this URI and then these new symptoms appear.

I figure I'll take her to the vet again on Monday or Tuesday. Loving our bi-weekly vet dates.

In the meantime, is there anything I could be doing to help her symptoms from this mystery illness?

The diarrhea could be from any of those things, maybe even from the antibiotics for her URI. I would at least call your vet about this today. Kittens really cannot afford to have diarrhea for long. They get dehydrated quickly and have very few reserves to fall back on.