Swollen Feet

Starting around Thursday Feb 18th at 11pm, I noticed my feet were swollen. The swelling is mainly on the top and the sides of my foot.  It ends right around my ankle/sock line. There was no pain, no discoloration, and no scratches for infection.  I went to the ER because I was scared.  They ran blood tests and a urine test, and EKG.  My kidney/liver levels were fine, as was my urine.  The EKG was also normal. The doc advised me to elevate them and reduce sodium, and referred me to a general care doctor for followup.

I went to my GP on Friday the 19th and the doctor saw me. I showed him my feet and he asked questions such as allergies, meds, and what not.  He could not figure out what was wrong with my feet.  He sent me home saying the same thing the ER doc said: elevate, reduce salt.  He told me to come back Monday for followup.

Back at my GP on 22nd, the swelling almost completely went down.  I mentioned to him that I had codfish the night the swelling started.  Previously, fish never gave me an issue.  He noted this, and he said it was possible I developed a fish allergy.  He said he would consult an allergist to see if it is worth pursuing further.  He let me go, saying don't eat fish just in case.

Recently, the swelling has returned, and I have not eaten fish.  I have not gone back to the doctor yet.  I have no known food allergies.  The doctors both ruled out infections, broken bones, kidney, liver, hear problems.  I am on one medication: Methadone 95mg, since August.  The blood and urine tests were clean.  If I elevate my feet during sleep, they start to look to return to normal; but as soon as I start my day, the swelling comes back.

Does anyone have any insight?