what other reasons would my period be late?

I'm late by 4 days and I did two pregnancy tests and both were negative, one on the first day of when I should've gotten my period and one a day after (clear blue and boots digital) plus I don't have any symptoms. I'm normally pretty regular and if I'm late it's only by 2 or 3 days or sometimes I'm early. Buh I've been having a continuous cramping sensation in my abdominal, it ranges from severe to just mild and it gave me chest pain. So I'm really not sure what's going on with my body.