Your hamster is a boy. The genitals stay inside the hamster for protection. When the hamster becomes aroused or excited they swell. You may notice this when you handle them because they are getting excited. This is nothing to worry about.
Mine did that... My GF was holding him, which resulted in swollen nuts... When I held him his nuts... Well.... Vaporized.... Haha!
Sounds like a bad case of over load, in which case it may need to release the pressure. Or the results could be catastrofic do to major blow out.
Maybe it is infected with virus, or swelling cause by hard mating cause it divirginized.
The hamster's genital area swells when it's hot.