Just got back from the vet! She felt her stomach, listened to her heart, and weighed her (shes 55g). She says that the skinny spine and swollen tummy are never good symptoms, but she looks well looked after and is very bright and active, so she sees no reason to 'help her on her way' yet, as animals often don't even know they're seriously ill until the very end. She says it could be a liver condition (which is eventually fatal but can be controlled with a high protein diet), fluid in the abdomen (again fatal, but she still has a little while), a tumour (once again fatal, but has a while left) or cystic ovaries, (which isnt fatal (yay!) wont go away, but she wont notice either). The vet said she could do scans and stuff, but since hamsters are so small, its unlikely that they could do anything about it even when it's been diagnosed. So for now, Wotsit is looking well, and will be eating lots of nuts, peanut butter and egg whites (for high protein), and hoping for cystic ovaries
Although this was obviously not a good diagnosis, im surprisingly happy as I went in fearing the worst. Thanks for all your help guys! Ill keep you updated