It could be many things, from poor circulation to an allergic reaction, though that will usually be itchy and uncomfortable too.
Poor circulation can simply be related to immobility - our bodies function best when we are moving around, as the leg muscles help to pump blood from there against gravity. If you are sat still the heart has to do all of the work.
Poor circulation can also make someone feel their legs are cold, so they sit close to a fire or put extra clothes on, then the extremities will flush with blood and turn blotchy with red patches.