Patch of skin after bicep, looks like a long line and a dent..

[CBaz44]

I'm not sure what to think of it. It looks like a long line of skin going diagonal from inside my arm to the middle of the arm. It looks like it could be a dent, but when I move the skin the line moves. Strangely, its easier to see with little light. It comes right after the bicep flex, so near the tendon. It doesn't really hurt, and I can still use my arm and lift weights just fine. If I push hard and feel over it, it feels like there's an indent though. Any idea what it could be?