Hamster has growing lump under leg?

My male hamster that I've had for a year and 7 months, has a lump that I'm concerned about. It showed up a month to two months ago and it's grown, but not a whole lot. It's under his "arm" and it's kind of squishy. I haven't tried pressing on it, as I don't want to hurt him, but it's definitely not hard. Abscess? Tumor? Some other kind of growth? He's acting completely normal and happy, eating, drinking, exercising etc but I just noticed it has grown some so I thought I'd get some opinions. Thx! Oh and it's not any weird color, it's under his skin so it's just skin color (pink/black).