Pelvic aches?

For the past two months, I have had an achey pelvis on and off. The best way to describe it is that it feels like a pulled muscle in my pelvis. I feel the pain (which isn’t horribly uncomfortable by any means!) when I stretch, reach, or exercise. I haven’t felt any aches when I’m just sitting or not moving around. Any ideas what this could be? It’s towards the right side of my pelvis most of the time, but I will occasionally feel the “pulled muscle-like soreness” on the left side.