If she's 1000g, its common for BPs to go off feed once they hit around that weight. Its called 'hitting the wall'. Not to mention its currently the breeding season, where most adults do go off feed as well. Some females like being cooler when they're developing follicles. So that could be it maybe. She's a bit young and small, but not unheard of.
Your ambient temp sounds too low. If you can raise the room temp, that would be the easiest way to keep her warm. Tubs aren't great at insulating heat, so maybe build a little box cover out of styrofoam or wood to help insulate heat(like, something that covers the top, back, and sides of the tub. Like a single level rack almost). Heat mats provide a specific hot spot and don't really change air temps. So its important to make sure all the temps are within proper range or she could get sick.
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