Then go get some Bene-Bac if you can find it. Gel is better, but powder will do. Petsmart/Petco may have it, or go back to your vet and get some. Give her a dose one hour after every dose of antibiotic.
Also, feed her every time she gets a dose of the Baytril (enrofloxacin). Get some Critical Care, probably available at the same places. See if she'll just eat a big glob of it, but if not, you'll have to hand feed. Post back her and let us know if you can find it, and we'll give you some handfeeding instructions.
Baytril will probably kill her appetite, but guinea pigs MUST eat all the time or they'll die. So you'll probably have to hand feed for several days.