I had a pet rat named Klondyke. Klondyke developed a huge lump in her belly too... The lump got to be so big that you could see a pink thing/ball coming out of her bottom. Even though Klondyke did not seem to be in pain, she would jump around and play with me and the 3 other rats, it still looked very painful. We had to put her to sleep. The vet said it was a tumor.

It is better to bring your hampster to the vet early on and get it removed... then too late.

Good luck with everything!