Why Do I Have Severely Sharp Pain In Knees Without An Explainable Cause Or Visible Difference?

I'm a 17 year old girl. I have pain on the inner side of my left knee (as well as pain in my right knee that is supposedly because of a cyst but it's a different pain than in my left knee). The pain I get feels like it's in the bone.. I'll get sudden - completely random - pangs of sharp pain in my knee and then my knee gives out and I can't walk on it. Any pressure at all kills it. I've sometimes simply turned on my foot and I could feel the slight twist of my knee and the pain is so excrutiating, I hafta stop everything and am holding back tears. I'll get the pain while walking, going up stairs or simply sitting and then I hafta hold my knee tight to try to make it feel better. I don't play sports, nor have I fallen on my knees. I've had knee pain for months... this particular pain on the inside of my knee has been around for a month or so, frequently more and more. I used to get lots of bruises on my knees and I could never recall why.. I'd bump into some things but I always seemed to have bruises (up to six months ago even!) even if I didn't.. if that could mean anything in relation to why I'm having this pain? But I haven't bruised my knees in over a month! So, I really really have no idea what is wrong with me. It hurts really bad too. Oh. And my knee LOOKS perfectly fine, no swelling or redness or anything! Any help would be so much appreciated. Thank you.