Prego or crazy!?


So my period was due the 25th of september however it has yet to show up. Therefore leaving me with no menstrual cycle in the month of September and now it's October and I still have yet to get my period. I have had cramps like crazy but no sore nipples and really really bad back pain . I took a pregnancy test yesterday and I got a negative! So now I'm wondering still why period is so late. Any of you ladies had this happen or can give any advice before I go to the doctors ... Please help ease my lol little panic..... btw I'm pretty regular and yes I had sex the 25th too give my period a both however I got caught in the moment and ... no protection was used yet the test was negative.