What Causes Delay In Periods And Negative Pregnancy Test?

hey, im a 19 year old female who is late coming on her period. me and my boyfriend had unprotected sex in the month of feb. i was due on the 1st of march. on the 1st i had really light bleeding mainly brown in colour. thinking it was just a slow start to the period i didnt worry. it had completely stopped by the 2nd. ive took 2 preg tests between the 3rd and today both negative. i do have some signs of pregnancy, constant fatigue, sickness after eating, headaches, mood swings and some cramps, but the cramps dont feel like period pain, there less painfull and becoming less frequent as time goes on, its turning into a almost heavy feeling down there, as if im on but theres nothin there. i only had my first proper period last month after coming off the contraceptive injection back in sept 09, it was normal and lasted 6 days. could i be preg even with negative results, or is it just my body getting back into it after the contraceptive injection?