Do you have a thermostat? (Not thermometer; this is a device that regulates UTH heat output) I don't think I've ever seen a chain petstore that tells people they need those. They're absolutely necessary to keep your snake from getting burned.

You probably won't get sick animals from Petco, but they're known for poor husbandry and a general lack of knowledge about the exotic pets they sell, so you probably got a very stressed out animal.