[souffle]

CB it says in the first post she was 6 months in September therefore she is coming up to a year.
Any change in behavior can indicate illness and I think the best idea is a vet visit for a check up.
The weight gain could in fact be something going on inside her like a tumour or bloating due to fluid or pus in the abdomen. The most common cause in a female would be a womb infection (pyometra) This is quite serious and your vet should be able to check and see is this is a possibility.
Don't use a heat pad. It isn't that chilly and if she has always been fine at room temperatures then it isn't likely to be that. They can easily overheat with heat pads.
Older hamsters can get more settled and sleep more but at a year she isn't really that old to be honest.
I would definitely have her looked at by the vets. Let us know how she does.