This one better not wait lah please go see your doctor or A&E now. Swelling in just one leg at 36 weeks can be a sign of something that needs checking urgently, like a blood clot, and you really don't want to take chances with that. I know it's probably nothing scary, but with pregnancy your body changes so fast it's better to get it checked straight away rather than worry at home. Let me know how it goes.
Please get checked with your obgyn