I've had a lump under my forearm for a few years, had it looked at by my dr., and he wasn't concerned about it, forgot what he called it, said his father has one somewhere else, and they're usually harmless. Said if it grew much, could look at it again. It has grown, but have not brought it back up to him because only think about it when it's cold because it hurts when I get cold. He said initially he could remove it in his office, and I may have that done in near future, just to eliminate the occasional discomfort and concern. But I live in the south/southwest so don't have a lot of cold weather. Also showed it to a plastic surgeon once, and he said he could remove it, but didn't seem concerned about it. Hope that helps you feel better.
I also have a lump under my arm skin, it is about half an inch big, a couple of inches below my wrist. It's on the under side of my arm though. I just noticed it a few days ago, I'm not sure what it could be so I've been googling it. Some have said it could be a ganglion cyst, an arthritis lump, and not to freak you out or anything, but a lot of people have said it could be a tumor. If mine doesn't go away within a few weeks, I'm going to get it checked. So maybe you should get yours checked too.