What Causes Delay In Periods And Sharp Pain At Lower Back?

Hi I urgently would like to know what is going on and I m getting very worried. Here s the scenario: My periods are not very regular. I do have it every month but the dates are rarely the same - it could be a few days or one or two weeks late or early. However, I had my periods on August 28th, and Sept 23rd. One week before my period on the Sept 23rd, I had really bad lower abdomen cramps, mostly sharp pains on lower right and painful lower back ache. This pain continued for about 2 weeks after my period ended. I had intercourse on 8th and 24th of October. On the 26th of Oct, there was a small amount of blood mixed with mucus when i washed myself and thought my period was coming. But there was only a light brown discharge for one day and I had slight cramps the next day. I still have not gotten my period and I had brown streaks when I wiped myself on the 14th of Nov. Tested on the 13th of Nov for pregnancy but it was negative. I wonder if it s implantation and if so, why am I getting negative results even after not having my period for the month of Oct?