There can be several causes of it and some can be serious and require urgent action and care.
These need to be ruled out by an astute physician with physical examination and relevant investigations. The common causes include
1.Hypoproteinemia that is the decreased levels of proteins in the body.
3.Obesity can also cause ankle swelling
4.Varicose veins can cause foot swelling but not on other parts of body.
5.Deep vein thrombosis
is usually unilateral that is can cause swelling on one side foot only. It can be ruled out by a color doppler examination
6.Lymphedema which is much less common
But specific to your case I will like to rule out decresed renal function which can cause it and swelling will be present in face and around the eyes also.You need blood tests for blood urea and serum creatinine to rule it out.
Another diagnosis to be ruled out in your case is an allergic reaction to an element in your enviroment which would cause such symptoms.
Congestive heart failure (you may also get a dry cough with this),Drug side effects like Amlodipine and other calcium channel blockers, gabapentin etc...can also be responsible but you have already ruled these out.
Therefore it is advised to visit a physician and get yourself investigated.Not all drugs contain sulphites