Degenerative myopia?

How can I tell if I have it or not? I'm 13 and my vision has been getting worse every year for just about the past 7 years. I believe there was one year when my vision actually got better, but I dunno. Last I checked, my vision was -9 and -7 in my right and left eye, and I know it's gotten worse this year. Do I have degenerative myopia?