I want to go on the pill!?

I've just got back from the doctors. I wanted to go on the pill but he said no and suggested I go on a diet.. I'm 15, 5ft4 and I weigh 7 stone, last time I checked, I wasn't fat.. My mum told me I just have to suffer with periods pains if I won't take his advice... WTAF?! I sortof wanted it partly for periods and to be safe.. What can I do? :S