What Causes Delayed Periods?

I had my period twice in April, April 5- April 10 and again on April 20- April 25. Both were normal periods. I had sex a couple of times before my next projected period, which was said to come on May 14. Now today is May 21, and I still haven t gotten my period. I m more than sure that my partner didn t ejaculate inside of me, and two of the sexual inter courses were protected. I ve been having cramps like my period is about to come on, but again no sign of it. What is going on? Are the two periods that i had April prolonging my period?