What Causes Late Periods With A Negative Pregnancy Test?

Hi, i had my last period on the 26 february and not had one since. I have done 7 pregnancy test all of which are negative! i have ovary pain, both sides and sore breasts. Just cant understand it! I have sent my urine off today to the path lab for a pregnosticon and C&S to see if it comes back with anything. Just baffled???? Helen