Even though your hamster is still eating/drinking, but is suddenly being nippy and is hunched over, it is possible that your hamster is getting sick. Being fat may or may not be a symptom - right now it's hard to tell based on that and the other two symptoms which exact ailment it is. Just monitor him/her and take him/her to the vet within the next 24 hours if nothing changes/he or she gets worse. Hamsters are good at hiding being ill, which is why their condition can unravel so quickly if nothing is done.
Hope he/she feels better!
Hope he/she feels better!