What Causes Persistent Abdominal Cramps And Back Pain Post Menstruation?

Hi, I have been feeling really tired and hungry all the time for the last 3 weeks, i thought i could be pregnant as i have unprotected sex with my boyfriend plus i was a week late to start my period. I did a home pregnancy test and it came up negative and a few days later i started my period. My periods are normally really heavy and i suffer with stomach cramps and backache, but this one was light but still lasted for 7 days. 3 days later in and in still suffering with the backache and stomach cramps and i have to wee all the time. Could there be a reason why?? Thanks