havent been on period for 3 months!!

please help i havent  been on my period for about 3 months and for about 2 weeks i have been constipated and sometimes got these realllyy ad sharp pains in my stomache i couldnt talk it hurt so much but then i was fine later on now i just have belly aches everyday but i dont know why i havent been on my period for 3 months!!! my period was never accurate always every  months then 3 weeks tthen  months tthen whatever but never 3 months! i am quite concerned and i need to get rid of m constipation because i cant have a day off on monday because i have a maths gcse 1 year early and i cant miss it and the scool said i have to be in everyday anyway because otherwise i would get fined up to £1000 i do get dstressed real bad and start hitting people and cutttting them and hurting them then i feel real bad and cry about it yes i have a sh*t life and self harm i am 15 as well but please help and i am 10 stone 5 pounds and female