Hello, i have ptsd i've had symptoms for over a few years but only recently diagnosed, i have serve joint pain in back and knees,could this b linked?

It's possible: There is a definite association (a link but with unclear cause) between PTSD and heightened pain sensitivity. The lower back would be an expected location for this sort of pain to be perceived, the knees less so. Generally, the way to go about this is to get checked out with a good physical exam, possibly X-rays (but maybe not at your age) then if negative for pathology consider the PTSD angle.