What Causes Period Like Cramping But No Period?

For a little over two months now I have had period like cramping but no period. (I am almost two months late) I did have some spotting a week ago but it was not even for a full day. I have been extremly tired, moody, constantly hungry, nausea on and off, and headaches. Had a negative pregnancy test and also went to planned parenthood where they also did a urine test which came back negative. They do however want me to go back to re test in two weeks. What could be wrong? Is it possible that I may be pregnant or is it likely something else? And should I be worried?