2 weeks early period, heavy brown bleeding but negative pregnancy test

well, im 18 and been sexually active with my boyfriend for nearly 7 months. weve never used protection. For the past 2 weeks ive hasd sore boobs and sensitive nipples. my boobs are so small but theyve grew abit and feel heavier. Im constantly hungry and thirsty but still crave something but i dont know what. Anyway, last night, i found that i bled a lot but im not due until nearly 2 weeks now. my periods usually red but it was thick & brown. its been like this the next day too. ive had cramping on the righ side of my belly and in my abdomen. I took a test cuz i was scared if i was havin a miscarriage but it said negative :s i dont know what to do or think. any advice?