What Causes Missed Periods With Negative Pregnancy Test?

I am a 27 yo female stable relationship of 5 yrs... We have lots of unprotected sex with bf/boyfriend last couple months he does ejaculate inside... b4 then had a regular 28 to 31 day cycle now I have not had a period since 12/22/13 it is now 2/18/14..... All negative preg test idk if I am testing too early..... My question is, there still a chance I am preg bcuz the past couple weeks I have had bad period like cramping and the runs..... Idk wat to think ??? Looking for advice thanx =)