What Causes Sudden Weight Gain, Swollen Ankles And Legs With Thyroid Being On Lower End?

I have unexplained weght gain, my ankles and legs are swollen, My wrists are too large for my watch. I had my thyroid tested recently and I was on the low end of normal so my dr. perscribed a thyroid vitamin. I exercise some and eat okay and drink about 50-60 ounces of water a day. I don t drink softdrinks and have a pretty low sodium diet. I have gained nearly 25 pounds in about 5 months. What do you think my next course of action should be? is there a certian type of specialist I should see?