I'm sorry your hamster is ill. It sounds like she has multiple problems, and they are serious. The wetness on her underparts and associated diarrhea is probably the result of a disease called "wet tail." It is usually fatal if not treated. If you are to have any hope of saving your hamster, you'll need to get her to a vet for treatment as soon as possible.
Since it's Sunday, you may not be able to find a vet until tomorrow. There are some steps you can take to support your hamster until then. Remove her food. Eating more will just trigger more diarrhea. She may not have an appetite anyway. Dehydration is a danger, and you need to do something about that right away. The best thing to do is to give her some unflavored Pedialyte (made for human infants) from a syringe. Pedialyte is available in most drug stores, supermarkets, and discount stores. Some convenience stores carry it, too. If you can't get any Pedialyte, give her lukewarm water from a syringe. Put it in the side of her mouth. If you put it straight down the throat, it can be aspirated into the lungs. Give her water or Pedialyte about once an hour. Your hamster must be kept warm, so the heating pad is a good idea. Just make sure that it is positioned so she can't chew any electrical parts. It's also important to keep the cage clean. These are all supportive measures, not intended to take the place of veterinary treatment.
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I hope you're able to quickly get some treatment for your hamster.
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