What Causes A Delayed Period After Stopping Birth Control Pills?

I was on the pill for two and a half months. Last month I stopped half way through the third pack as it was making me feel ill. A few days later I got a period. This month my period is late but I am getting on and off menstrual cramps. Chances of being pregnant are very low but I took a pregnancy test anyway and it came back negative. Is all this because I came off the pill ?
For your information I have always had regular periods. I also have severe period pains throughout the years, and am due to have an ultra sound to check that everything is alright in that area.