She started taking them about a week ago for an infection, and now, right before she gives herself a bath, she...hisses. Not like an angry cat hiss, more like a velociraptor with the volume turned down. My guess is that the medication is giving her dry mouth, and that's the sound she makes trying to lather up spit. Am I right, or could this be something more serious?

Ya your probably right or unless she is having a bad reaction from the antibiotics

Yes it could be a bad reaction to the "drugs" i would say take it to the vet and ask him/her if you should switch medication or maybe your cat is just getting old like mine... So the best thing i can suggest is to take it to the vet, do it right away you don't know what the medication could be doing rong.