I had a quick read of some veterinary sites, and here's what I could find:
Bumps and spots on hamsters are pretty common. In most cases, you're dealing with an abscess - a hard lump that forms around a wound or cut that hasn't healed properly: The body's way of preventing infection from spreading further.
Just like in humans, there is also a chance that a hard lump or bump on the skin could be a cancerous tumour. Unless your hamster has been acting strange, not eating, and looking lethargic... I think you're safe.
It could also be a skin condition of some sort, essentially similar to a zit on a human.
These are just some possibilities based on 5 minutes of online research by a layperson though, so my advice would be to take your hamster to the vet just to be sure.
