For the record...Benadryl is an antihistamine, not an antibiotic. Regardless, it should only be used under direct supervision of a vet, as stated previously.

If you plan on keeping this rat, I would have it seen immediately rather than "waiting to see how things go". Small animals deteriorate very quickly, and things can go South in the blink of an eye if medical assistance is not provided at the right time. Many times, small animals come in too sick to save, and usually by the time they're showing a few symptoms, it's too late.

So hopefully, if you get this kid to the vet sooner rather than later, you can nip the possible resp. infection (if that's what it is) in the bud...While you're at it, bring a fresh stool sample for analysis.

Good luck, and yeah, she's really cute!