What's this pebble under my skin?

I'm 28, but for over 10 years now, I always feel a little pebble of maybe around 2 or 3 millimeters diameter under my skin, anterior to my elbow. I press on it and it feels like a mini marble, but I can't tell what it is. I also have another under my axilla. And the last one.... tmi. Anyone knows what it could be?