Any Dr's on Answers?

just a thot.....my GP think becos am young (22yrs) am invincible so he tells me off whn i go to see him so was wondering if anyone could help.....my knee hurts, i cant sit for longer than 20mins i have to get up and walk for a gud 5-10mins. I have had this before when i was doing my GCSE's like years ago (i just thought it was revision stress), but it was never this bad, if am sat for about an hour i find it excritiatingly painful to move my leg...hav i got arthiritis? can i get that at 22yrs of age??