Are you doing anything to cause her to pant? Like playing with her hard with an interactive toy? Is it hot where you are right now?
If the answer to both of these is no, then a vet visit is in order. Panting can be a sign of a heart problem, asthma, or other lung problems. I wouldn't rush her to the ER clinic, but I would call your vet first thing in the morning and take her in tomorrow.