What Causes Delayed Periods With Negative Pregnancy Test Results?

I have been on birth control since I was 16yrs old, I'm 25 now. I stopped taking it about 2 months ago and my boyfriend and I have trying to conceive. We have had unprotected sex a couple times. I did have my period in April, but now I have not had my period for the whole month of May. I took a HTP this morning and it was negative but I always feel I take those wrong and they dont work properly. Should I test again in a couple days or just wait and see if I get my period in June? Could it be something other then pregnancy?