What could be a cause of heaviness in legs?

crabs

There are quite a few things such as sprains, strains, blood clots, cancer, varicose veins, tendonitis, to name a few.

Poor circulation perhaps? what would cause my legs to feel heavy and have trouble standing up from a sitting position

Muscle stretch could be the main reason for pain and fatigue in feet. Over workout could too causes pain in legs and fatigue. Lots of jogging can cause pain in legs too.

You might want to research RLS (Restless leg syndrome)