Sounds like you just sprained a muscle probably from twisting your knee or landing badly on it. Its not uncommon to be sore or hurt themselves, usually you cant hurt yourself too badly. Dont let them injury stop you from practicing your front side, and remember to keep some speed up while making those turns!
I get knee pain myself, or atleast used too due to not bending my knee's enough and being too stiff..all the bumps would just kill my knee's after 3-4 hours of riding. More forward lean on my bindings + insoles fixed that..
'09 151 Never Summer SL-R
'10 K2 T1 DB Bots
'09 Burton Cartel Bindings