Hi RiAnne,
You are too young to have to deal with lower extremity edema like this! I would not settle with a doc stating that they don't know what it is. If your primary can't tell you, perhaps he/she can direct you to someone who can.
My initial thought was Peripheral Vascular Disease (PVD) or some type of lymphatic disorder. You are peeing so much on the water pills b/c they are working, but it is probably a pain to go so much. When elevating your feet, to be really effective, they have to be above your heart.
Good luck. You are your own medical advocate so don't give up on getting an answer

Have a blessed day.
Sarah