Thin endometrium????

Okay so AF was supposed to show up on April 24, and nothing still.. Two tests and they are neg.. No cramping.. I did have a sharp pain in my right side so i went to the doc and he did a abdominal ultrasound and said my ebdometrium was thin and doesnt look like i am pregnant.. But never said how thin it was.. Does the endometrium lining thin when you are supposed to have AF?? Or what can anyone give me insight please..