Age: 15 Started: 2006 Number of times to date: Only 3, am I ok?

I'm 15 years old, and I started having my period on August 29th 2006. Up to this date, I've only had its 3 times, and they've basically been a year apart, my last being June 02 2008, and the one before that, June 04 2007.
Today I went to get an ultrasound, because my family doctor suggested one. It was really weird too, me being only a teenager going for one of those :-P
My last family doctor said it was normal for a girl my age to be having gaps like these...
but is it really? Please give me your opinion!