Sweetie I have to be straightforward like Becky and warn you this likely isn't going to end well. Healthy female rats don't pant in labor, they normally give birth with no muss, no fuss and very little blood or mess. It's possible she's panting due to a hot ride home in the car though, rats don't do well under high heat conditions so first get her settled in a cooler spot, out of direct sunlight and alone with food, water and clean bedding. Watch her closely, if she appears to stay distressed and struggling put her down humanely. If she recovers then just let her be, feed and water her and just keep a close eye on her. Even if she does survive, oftentimes female rats found heavily pregnant at the pet store don't have successful litters as they are not in great shape in the store to get preggers and are stressed and fighting for food and water during the time they are carrying. I don't want to worry you but feel I must be very honest in that this just isn't a situation that sounds good at this point. Please let us know how it goes.