I've missed my period for about 6months is that bad? ?

Im 15years old and I had my last period sometime in December last year(2010) and I still haven't had my period meaning I haven't had it for around 6 months. I know I'm not pregnant because i haven't had sex, I'm still a virgin. Is it bad that I've missed it for 6 months? Should I go see a doctor? Is it normal? Is it healthy? My mom thinks it's not normal and it's not healthy. Please help!! I was a gymnast for 3 years and stopped. My friends say it okay because I was a gymnast. But I don't know. Please help!!!!!!! Thanks!