Hamster has a sore...?

I had the Habitrail Ovo cage, hated it so I got a new one. My hamster is doing better now in the new cage, he's exploring more, eating more, and drinking more. When he was in the Ovo, he stayed in the tiny hideaway for so long, when he finally came out there was a sore about the size of my fingernail on his back near his side. I'm wondering if it was from sitting in his own pee / getting wet because the water bottle leaked. He's eating fine and being active so I think he's alright, but this sore is worrying me. What should I do?