What Causes Swelling On Calves And Behind Knees?

I have had chemo therpy for breast cancer and radiation treatment. During radiation treatment I could not swallow food and following treatment my THS was 25 and synthroid 112 mg was prescribed. I have swelling in both calves and on the back side of my legs behind my knees. Is this swelling due to thyroid or lymphatic system being messed up due to cancer treatments?