Young Male Syrian with a growing belly...?

[Penn]

I recently adopted a young male Syrian from a Pet rehoming website. (I know these websites tend to be awful, but I'd lost a hamster the week before, all the local pet stores had no males, and I am not blessed to have a breeder near to me.)

He's been with me for just over a month now, and he's come out of his shell - bit my fingers twice when he thought they were food. His name is Jasper.



He has definitely grown since I got him but the past week I've been concerned at the size of his stomach. I always rib-check my hamsters for their weight (Gently "pinch" around the middle, if I can slightly feel their ribs, they are an ok weight in my experience). When he first arrived he was very slight and of course, he has put weight on, but his stomach is firm when I press (it doesn't seem to hurt him) and looks like he's swallowed a ping pong ball. It's all over (so not limited to one side the way a tumour would be) and as far as I know he's still peeing and pooping just fine!

He's not been given any treats that my other hamster hasn't, the only recent addition to my hamsters diets is plain popcorn (as a treat, no more than three times a week). Could this be the cause?

Jasper is DEFINITELY a male because, well those testicles are hard to miss =p

But other than assuming the worst, I have no idea what this could be. Does anyone have any thoughts?