Personally I would call your OB/GYN right away for advice. But here's a women's health link about blood clots during menstruation.
"Normal Blood Clots During Period
During your period, your body will release anticoagulants that keep the blood fluid and thinned. However, if your flow is extremely strong, the blood may pass before the anticoagulants can work. In this case, the blood may clot as it is passed.
However, there are times when menstrual blood clots should be a reason for concern. If you are passing clots that are bigger than a quarter on a regular basis, it could be a sign of something else.
In this case you should consult with your doctor immediately to find out the cause. The following conditions could be causing you to pass larger than normal clots.
Hormonal Changes Can Cause Blood Clots During Menstruation
The menstruation cycle can be affected by fluctuations in the hormones progesterone and estrogen. These hormones control how the body sheds the lining of the uterus. When these hormones get out of balance, the wall of the uterus can become very thick.
Once this occurs, you may experience a heavier flow, and more menstrual blood clots. Hormonal changes can be caused by a number of things including menopause, excessive weight loss or gain, or as a result of some medications. In this situation, your doctor may put you on a hormone therapy."
I wouldn't panic just yet, but I would be sure to call your doctor and see what her advice is.