Is it common to get a lump on your forearm it comes up when I flex, kind of soft, but doesn't hurt. What could it be?

Well, it could be just a muscle... but know way to know.

Can you feel the lump without flexing? If not, my guess would be a muscle (I have that on the top of my forearm about 3 inches back from my wrist)... wouldn't worry about it...

But to be safe... it can't hurt to go to the doctor :)