You haven't been using your tendons (Muscles that make your knees bend) often enough, so your heart has not beeen pumping enough oxygen normally for them to run, too. Now that you have used up most of the oxygen in your body that is supplied for excersize, your body has started the process of fermentatio nwhich causes lots of lactic acid to build up; Painful and stiffening.Your legs should get better if you just don't push them to hard while they are liek this (No running, jump rope etc) but if you continue to jump rope regularly after this episode, your legs will shortly get used to producing enough oxygen for them to excersize without fermentation.
In short, take some pain reliever and lay down, don't run or jog or anything.