What Causes A Delayed Menstrual Period With Stomach Cramps And Nausea?

I had a very weird period last month, a few days late and then started out brown for a few days then a regular period for about 1 and a half days, then back to brown. Took a test it was negative. This month no period about a week late, pregnancy test said negative but I had all the symptoms like I do before my period, just no period when it was time. I still feel nausea throughout the day and sometimes cramping like I am gonna start. What is wrong??