HELPPPPPPPPPPP with my period?

well.. the first time I got my period was 5 years ago when I was 12 ..I found out then I put a pad but nothing else appeared it appeared just once ..then after six month it came again but normal this time it stayed for 6 or 5 days but since then it doesn't come at time ..maybe after 2 months , 4 or 6 and I think it came only once after one month or the previous periord.. well i'm fat and my mom told me that this maybe because of fatness ..and doctors said on the tv when another girl asked the same question as mine that she has to have a diet first then see the results.. oh yeah when it came on time ..at that time i was thin cos I made a diet and lost 15 KG ..I'm afraid to go to a doctor ..please help and tell me if you knew about my condition..I'm 17 now turning 18 :S Oh yeah and I have very small ..maybe the hormones aren't stable so that's why?