it is not uncommon to have inflammation in your knees after any kind of workout (cycling, running, hiking, etc). is the pain just a dull ache or is it really UNBEARABLE? there could be many things going on...but i'm guessing that since you just started riding your body isn;t accustomed to the stress yet. do you stretch? how hard are you riding? are you spinning or mashing? are you using spd (clipless) or platforms?