Why am i bleeding on birth control?

Alright, so i just started my birth control pill. In the middle of the 2nd week i started bleeding. My mom said that it's just spotting. but it's really bothering me because it's heavier than it should be. and i've been like this for 3 going on 4 days and it's not going away. it seems like i have my period, when's this going to go away? i'm also getting really bad cramps. i went on this pill to get rid of my dier cramps. but now i'm getting them so bad i'm in bed crying. i know my body is trying to get used to the pill thats why i'm bleeding but whys it so bad and when will it stop? please help me!