What Causes Blood In Urine?

Hi I am 32, I started my menstral cycle 2 weeks ago, finishing a little over 1 week ago. I have not been sexualy active since 2 Feb 2010 and as far as I know I am not pregnant (would know by now) periods are within a day or two regular.

On friday (expectadly due to ovulate) I went to the toilet and found a small amount of blood and a Blood clot the size of a 20 cent coin. I freaked out and well there has not been any other sign of blood since. Am highly confused and worried.