Question about menstrual cycle and fertiltiy?

I am 27 years old, and have been trying to get pregnant for about 4 months now. My cycles are long and irregular, varying from 33-40 days each month, making it very tough to pinpoint ovulation. My last period started on April 30. Today I am experiencing cramps and just wonder if anyone knows why I might be cramping so soon before my next period is expected. Any input is appreciated. Thanks!