Could sharp pelvic pain have been a ruptured ovarian cyst?

On the third day of my period, I had a sudden attack of really strong pelvic pain and cramping and bright red vaginal discharge with lots of little dark clumps (normally it's really dark red). The cramps felt like 9cm dilation during labor! My family took me to the emergency room and after a pelvic exam and ultrasound, the doc was left scratching his head - he said I didn't have any cysts, and the blood pregnancy test came up negative. I refuse to believe it was nothing - it was horrible and very real! Could it have been a ruptured ovarian cyst? Anyone else have an experience like this?