What is wrong with my knee?

I have had an on going problem with my knee where it keeps clicking and aches all of the time and excrusiatingly painful when I excersise. Sometimes after excersise it swells up and is very bruised. In the past week, whenever I run or climb up or down lots of steps or nearly anything puting pressure on my knees I faint.

Because of my knees I am having other injures such and spraining my ankles and pulling muscles in my legs.

I have not been diagnosed with anything for my knees or fainting yet

I am I fourteen year old girl and am very active and sporty. I am average weight and very healthy.

There is no history of fainting or knee problems in my family. There is no history of anything like arthritis either.

Thanks for any answers