[cypher]

Do be careful with the heat pad as it's easy to overheat them, it's always best to keep the room temperature up rather than using heating close to the cage if you can.
If she had been going into torpor then she should be back to normal as soon as she's warmed up again so I doubt it is that, they can be a bit less active over winter but do still normally come out at night if the room is warm enough
Have you had her out for a full health check?
It really is hard to tell what the problem may be, how long has she been like this?
They are very good at hiding signs of illness but if she's eating & drinking ok there may not be a problem, I would check her over just to be sure though.