What Causes Delay In Periods,stomach Pain And Diarrhea?

i'm about 3 weeks late on my period. me and my boyfriend use the pull out method. i've taken like 6 preg. test and all come out as negative. i've even bought the exspensive ones. my stomach has been hurting and all the symptoms of my period is there but no period. and i have had diareah alot. i'm just so confused, i need answers.