What Causes Vaginal Bleeding And Stomach Cramps?

Im 16 and I had sex without using any contracpetion on 26th and 27th February, my last period was the 11th of Febraury and i am not due my next one yet, however on 7th March i started to bleed, not as lightly as everyone says it should be, but not as heavy as my normal period, my breasts are tender and soare and i cans sometimes throughout the dayhave horriblle cramps in my stomache, im not bleeding as much as i was on monday its very light now, could you give me some help and advice as im very cofused.