I've missed about 3 periods someone help?

I'm 15 and started about a year ago and since then have been EXTREMELY irregular , but my period has been gone for about 3 months now and i'm getting worried. I weigh 99 pounds and i'm about 5,6 i havn't got eating problems or anything like that , i love the food i'm just naturally like that and i've been this way for a while and still had periods and stuff so i doubt its that . I'm not stressed so it can't be that and i'm not sexually active, i'm wondering if its just irregularity , but i'm still getting worried please help me xxxxx