Why didn't I get the flu this year? (NOT a swine flu question) Really need help please !!?

I'm 16 years old. I don't really remember having the flu before 7th grade, but when I was in 7th grade I got the flu REAALLLY bad and I was out of school for 2 weeks and I ended up in the hospital for a few hours. I also got the flu again, so I got influenza A and influenza B that year. In 8th grade I got the flu and was out of school for a week. It wasn't as bad as 7th grade but still pretty bad. In 9th grade I got it again. In 10th grade I got it again and I ended up in the hospital again for a few hours. Each time I got it, it wasn't as bad as the first time. Anyway, why didn't I get the flu this year? I have never gotten flu shots. I know the virus mutates but did I build up an immunity to the base of it?