Sudden pelvic pain after sitting up really quickly?!?

I was sitting in an office chair, slightly reclining, doing homework- and I sat up suddenly and had this incredible piercing pain in my pelvic area. It felt like I had a thick wood stick inside me that was suddenly snapped in half. I am a 21 year female. I am not on my period (and not pregnant), have no health problems. I got up to go to the bathroom and afterward the pain went away within a few minutes. This has happened maybe once or twice before in the past maybe 4 years. Does anyone have any ideas to what might have caused this pain?