Hi there, I don't know if you happened to see the thread I started the other day or not, but I just took my baby to the vet yesterday for this very same reason (sudden panting). My thread is here:
http://www.yorkietalk.com/forums/sic...-read-now.html - there are a few updates from me throughout the thread, so you will just kinda have to skim around.
I may have missed it in your post, but what antibiotic is your baby on? I would definitely get online and look up the side effects of that specific medication. Panting could certainly be a side effect. I took my baby in yesterday to the vet because she was panting, and it turns out that she actually has an infection in her chest. So the vet put her on antibiotics (Clavamox) to help clear it.
Is she panting when she is indoors too, in the cool A/C? She doesn't just pant when she is outside in the heat, correct? If so then you know there is a problem. That's how I knew. Tatum would pant if she was running around outside and it was hot and she was tired/exhausted....but she never panted when she was inside in the cool air, and there was no reason for her to. So I knew good and well when she started doing it the second time (the next day after she started the first time) and she was indoors, that there was definitely a problem. Something was going on because she was in no way "physically" hot. So if your baby is doing this as well, then I would definitely be suspicious.
It could very easily just be the meds, maybe it is a reaction that she is having to it. Could be a side effect from that medication. I would look that up online for sure. If that medication doesn't list excessive panting as a side effect, it could possibly be an infection somewhere. The vet listened to my Tatum's chest and heard lots of rattling....turns out she has an infection.
I would definitely get it checked out and best of luck to you and your sweet baby.