I've never experienced heaviness but in my first ever LD I flew and now I can do it every time (can't always stay in flight! ). When you become Lucid you've got to have the confidence that you can fly no matter what. Try running as fast as you can and then jump off a cliff, imagine your self gliding and you should stay level or float to the clouds!

I think next time you have a LD you won't feel heavy, perhaps that was a one off thing. GL