I've Been Having Pain In My Knees For A While

I've been having pain in my knees for a while now, and they're tender to the touch, when sitting with legs streched out usually causes shooting pain, any way I sit actually. I'm not sure what's going on. My right knee gives out sometimes when walking. I've never had this problem, pain before, until a few months now. Makes it hard to keep up with my 4yr old.