I’ve recently got a baby hamster, he’s about 10 weeks old.

Hi, I’ve recently got a baby hamster, he’s about 10 weeks old. I’ve noticed his bottom area has been looking a bit sore. I called the pet shop I got him from, and they told me it was just his testicles growing, I’ve had male hamsters before and don’t remember them looking like this. Today it’s changed colour and has gone dark brown and his tail looks dirty. He’s eating and drinking and seems happy in himself, I’m not sure if it’s wet tail or if he’s just not cleaning himself properly. He goes to toilet in his little bed rather than a corner of the cage. I have got photos of how it’s changed as well
JA: Hi there. I'll do all I can to help. Has the hamster been licking or rubbing the sore?
Customer: hes been trying to clean it, he’s been taking little brown bits off it, but only when he’s grooming, otherwise he’s leaving it alone, he doesn’t smell either, which is why I’m unsure if it’s wet tail
JA: What's the hamster's name?
Customer: Honey
JA: Is there anything else the Vet should know about Honey?
Customer: no, not that I can think of