My period is really irregular... is this normal?

I started my period about... summer of 2007 and since then I've never had a period like clockwork. There's normally a gap of about 2-3 months between my periods, but recently, I'd had my period around July and only came on towards the end of November... The thing that's brought this up is mainly because I got a letter from the doctor saying I need to get my Teen jabs and there was a questionaire enclosed and one of the questions was about when my last period was and I'm not the most open of people to talk about things to strangers so I thought I'd post up a question... Thank you :)