Lower abdominal cramping, back pain and no period yet for a week?

Help what's going on, my period is late which never happened before. My abdomen is experiencing cramps and my pelvic area is aching. PMS started 2 days before my period was supposed to arrive but nothing came except for the pain and cramps a couple of days later. I thought that I might be pregnant but I'm not all HPT tests came out negative. I'm frustrated can anyone tell me what's going on?