Why do my knees hurt?

[MaHaLa]

Hey there! This is my first post here although I have visited this forum lately.
I ditched my shoes 8 months ago and everything was fine until a month ago.
Whether I ran 2k or 17k my knees began to hurt. Having already signed up for my first half marathon i completed it in two hours without any pain! (Aptil 26th). Now, the pain is back, and it doesn't look like it's gonna disappear...

The pain is in the lower knee cap. I've had jumper's knee and the pain is just above it. I'm 18 years old and I believe it's a bit too soon to deal with bad knees.
Can it be a form issue?

 

[DNEchris]

Can it be a form issue?

Quite likely - any time I've found my knees hurting I've found the pain went away when I shortened my stride and increased my cadence.
Try it and see what happens!

 

[MaHaLa]

I think you're right. I tried it yesterday but a strange thing happended. It felt like I had found my spring (I bended my knees more and took more steps - and it felt awesome).
Then, three kilometers into a 5k I felt pain in my knee. When I got home, the lower part of the knee didn't hurt at all - My upper part did...
One thing that I forgot to write in the first post is that my ankles have been very stiff which occured almost at the same time as the knee problem.