Why did i self harm? please try help me!?

So i use to self harm because i had things going on in my family and my mum helped me through it and stuff and i hadnt done it for about 8 months. Everythings sorted out now but i went back to school the other day and i just got really angry and embarrassed and i dont know why but when i got home i started hitting my self and ive got a huge bruise now and i dont know why i did it, i dont show it at school because im terrified of someone seeing it! Please help me, why have i done this to my self? i have nothing to do it for ?