What Causes Delayed Periods Along With Abdominal Cramps?

I have had my period in January 2weeks late... After the 1st week, I took a pregnancy test that came out negative, so I waited and it came a week later. Now, I haven t since my period since then, I m having cramps and stooling, yet my periods have not started. I have not been sexually active so I know I m not pregnant. my period is late by a a month