I think my hamster is sick. He has weird bumps on his back and a weird rash thing on his underside

Have you recently changed the brand or type of bedding you use?

How old is the hamster?

Unfortunately lumps and bumps in an older hamster are not a good sign. Accompanied with the rash though it could be environmental or even dietary.

A lot of pet store wood shavings are actually not very good for rodents, I had trouble with the scented ones with my rats, make them sneeze constantly so I had to switch.

Sounds like wet tail and needs to be treated by a vet.