I think it varies...

I have PN in both feet. Only my left swells.

Swelling in the ankles can be a general medical systemic cause.
Heart failure, diabetes, kidney problems (nephritis), drugs like gabapentin etc...will usually cause equal swelling in both feet.

One foot implies something is going on there.

1) swelling can reflect something in the abdomen...swelling. Lymph nodes, ovary etc.

2) If the swelling goes down overnight and then develops during the day, it points to a circulation problem.

3) swelling that stays, may be gout or some form of arthritis.

4) my swelling is related to the surgery I had on my instep. I can minimize it with tricks, like laser pen, cold packs etc. Some days it does not manifest at all. Lately it has been much better with the experiment with the laser pen.
Some surgical procedure or injury to that ankle/foot may disrupt the lymph return to the veins in that area. This is called lymphedema.

So there are many things to consider as causes.

If you are a diabetic, then it could be a condition called Charcot's foot. This would not likely go down overnight like a simple swelling would.

Patients with Reflex Sympathetic Dystrophy (RSD), have swelling in the affected area very commonly. This is another form of neuropathy affecting the sympathetic nerves. They have also swelling, discoloration, and sweating of the affected part.
RSD is typically accompanied by severe pain...worse than PN pain as a rule.

So start by keeping a log to show your doctor. When you have swelling, when it is better, if at all, or what brings it on. This will help pinpoint what is going on.