i'm not doctor, but i've had that my whole life and i always thought it was scars from when i had chicken pox when i was 8.. but then my sister started getting it too and both of our doctors said it was due to poor circulation and it very common.
it can increase your risk for blood clots, so if you spend a lot of time sitting or work at a desk job, make sure you get up and move around every once in a while.