Hi!
If you were my patient, I would advise you that what you are describing sounds like a blood clot. It can be normal to have these during your period if they are not larger than about the size of a nickel. If you are having lots of big ones, it suggests that you are bleeding a little more than we would normally see, and I would suggest a trip to your doctor.
If you just have a few small ones--no worries.
Good luck!
Dr bB