Sensations during astral projection are unique to everyone. Common tropes include hot/burning, electric/tingly, cold/wet ...
Sometimes, when you're completely immersed in your projection, you can experience the same sensations your astral body shares as it traverses the plane. Sometimes, the sensations don't correspond to the circumstances of your session, but that's normal too; the plane works on nonsense logic. You could be freezing cold in a desert, or sweltering hot at the top of Mount Everest!
As long as you aren't in pain, there's nothing to worry about.
It could be exactly what blackbeldem suggests but just another idea I had, what position are you adopting to meditate? It's possible that you're accidentally obstructing a blood vessel or nerve which may make your leg feel warmer, if that's it then just try a different position and see if it remedies the problem.