Guinea pig's pooping mucus?

About two weeks ago, my boyfriend and I bought two baby guinea pigs. A couple days ago they were brought to a vet and diagnosed with URI's and were both put on Baytril. Today I notcied that their stool is not the same. Mitzy's poop has dried mucus on it, and Maggie pooped straight mucus. This isn't normal. Could this be a side affect from Baytril? They've only been on it for a day now. They're both still eating great, and drinking great, and still have mounds of energy. What's wrong with my babies?