What's happening to me.?

I'm having mood changes mad happy mad the sad and then happy agian.......it literally is bad to when I'm mad or sad wanna rip someones head off then wanna slit open my wrist till I bless out and never wake up. And plus physical injuries j can't explain my knee is in really bad shape but they doctors say there is noohing wrong but if nothing was wrong I would yelp every time I stand up or limp after sitting longer than an hour....and I'd ask my family for help but they think I exaggerate everything And don't wanna listen... But I saw this thing on tv(bones) and this guy had been really upset and had hurt his arm and when he was helped out his arm was better and the shrink on the tv said that the way u feel inside when not capable of being released will express through UR body such as leg and knee(ECt) pain....is that what is going on with my knee...oh yah and whats with the mood swings...please I'm only 13i wanna live my normal life where I can run with fiends and be. Happy or at least have better reasons to be upset...please help