What Causes Delay In Menstruation Other Than Pregnancy?

Hi my name is Dana I have missed my period for 18 days now I have never missed my period before besides when i was pregnant with my 1st child, i have had 5 tests done though an all were negative an 3 days ago i had a blood test done and it was negative so im not pregnant, i have graves disease and i dont know if that could be why im not having it, but i started having pains in my ovaries yesterday and they persist to get stronger and stronger, is there anything i should be worried about?