What Causes Stoppage In Menstruation?

Hi doctor, My girl friend had her period onJuly 3rd n we had unprotected sex on July 18 on the same day she had postpone 72 and her next cycle is on July 31 bit still there is no symptoms of period, she had some different pain on her lower stomach from today August 1St.is she pregnet or please advice what need to do. Guess pill didn t work.