What Causes A Painless Blister On The Leg?

i have a air bubble like bump under my skin on my leg what is it? Ive had a bump about the size of a quarter on my left leg since i was about ten, like right next to my shin on the muscle part, its only there when i squat down on that leg and when i push on it i can feel like air bubbles, it has never hurt i though about like poking it with a needle to let the air out, is it air or wth is it?