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I think she needs seen by the vet. It could be a number of things but the weight gain is a worry as this is likely fluid retention or bloating. The most common cause in females is a pyometra or womb infection. This can be closed where the womb is full of pus. To survive she would need spayed to remove the infected womb.
Heart problems can cause fluid to gather and so can kidney and liver problems. Some of these are treatable short term to make her comfortable.
A slight stroke could be making her less active and an antiinflammatory can help recovery.
Get the vet to do a full check including teeth and feeling the abdomen carefully for lumps as well as the womb for swelling.
Hamsters retaining fluid tend to feel squishy and not firm and muscular. Is she drinking more would you say?
Don't let her meet the dogs - it will stress her and make he more prone to problems. No matter how much you trust them you never know with dogs. Keep her in a separate and quiet room. Hamsters hide fear well but it releases stress hormones
Hope you can have her seen and she can be helped.