That happened to me when I was 32. I bled for a few weeks. I went to my GP (I was new to the area and didn't have a GYN yet), and she ran a battery of tests. Everything came back normal. Once I stopped bleeding, she did a Pap smear. Everything was still normal. Soon after my cycle became more regular. Two years later my oldest was born.
To this day I have no idea what happened.
I'm now 42, and I haven't had a period for 9 months. Before that I was getting a period once every 3 months. My OB/GYN has done tests and an u/s, and everything still looks normal. My uterine lining is thick, but no abnormal growths.
I read some of the previous responses saying it could be fibroids or cancer. That's probably true. But it could also be nothing. Try not to worry. You will know more soon.