I noticed tonight when I was idly rubbing my calf a little bump that popped back down when I moved my leg. I figured out that I can pop it back out by rubbing a certain way, and it pops back down when I push on it or move my muscle in a certain way. It looks elongated like possibly a little loop of vein or artery? It's deep under the skin, there's no skin markings. No pain or any symptoms, except it hurts a little for a second when it pops back in if I push on it. Does anyone know what this is? It's not near the bone, it's on the back. Some kind of vein that can prolapse between muscle fibers? Is that even a thing?

Edit: Pictures http://imgur.com/a/cbyEf

Picture 1 is popped out, 2 is back in. I had a hard time getting it to pop out as much as it was before, but I did get it to pop up slightly and then go back in. It feels like a vein or something ropey.