What Causes Severe Headache And Irregular Menstruation?

I usally get my period for 3 or 4 days , i missed it last month and only got it for one day this month , and i am having tummy pains and a really bad headach , my roommate says could be a miscarge , but i took like 4 pergnacey test and one test at the doctors they all said - , ? any other ideals as to what it could be ?