I don't have my period every month :/ Is that bad?

I rarely ever have my period every month. The last time I had my period was 4 months ago. My period stops for a few months, then I get it back again, but the thing is.... I don't know when I get it. Why don't I get my period every month? Is it because I'm overweight? I'm 15, and I've never had "you know what", so I can't be pregnant. And this started 2 years ago, so I am DEFINITELY not pregnant. When I was in grade 6, my period use to be monthly. Then when I started high school, it started to come every few months. Some of my friends say it's normal, but I don't think it is :( Can you help me please? I'm concerned Thanks in advance :)