Why does my back hurt when I do crunches?

if i do crunches, after I've done about 15 of them, my back starts to really hurt. Its nothing to do with the floor being too hard and i think I'm doing it just like it shows online, but it still hurts. Is it something to do with my back muscles being exercised, or am I arching my back. Please, I want to exercise my abs, but I'm afraid that if I do too much the way I am now, I'll do damage to my back so if you know why I would be felling pain when I do crunches please tell me.