What Is The Treatment For Stomach Pain And Heavy Bleeding?

i am sixteen years old and my period is now two weeks late. i am sexually active, however always use contreception. i have took two home pregnacy tests just incase which have both came back negative. my periods are fairly regular and are either only ever a couple of days early or late but never this long. i have been having some stomach pain now and again but i am now very worried and confused.