I believe you should be worried until you rule out any possible dangers when you visit the vet. Hamsters are easily prone to cancer and the vet should examine your pet and advice you on the best course of action to take.
If it's squishy and you can move it around a little then it's probably just a fat deposit. All animals have them, my cat had one on her back. If your hamster looks like he's in pain because of it, or the lump is hard and not in a place where bone or cartilage should be then take him to a vet. If you still feel uneasy, even if the lump is squishy, still take him to the vet, better to be safe than sorry.