If it feels like you have been sitting cross-legged for too long and the knee feels weak and 'loose' then it is quite likely you have overstretched a ligament by sitting in an odd position. However never trust a diagnosis over the interweb and go see your GP - many GP practices these days have an attached physio so they may be able to get you seen much more quickly than a hospital referral. In the meantime try not too stress it too much - you'll miss your knees if they go!