I think this is more difficult to understand than the doctors or we want to admit. Here are a few 'facts' to consider.
It doesn't matter how many nerves are in your body. MS damage happens in the CNS (brain, spinal cord, optic nerves, cranial nerves) NOT in the peripheral nerves. In other words the tingling probably isn't even actually about anything in your foot. It is in the way the signal travels (or doesn't) to and from the brain and how it is interpreted along the way.
Your body works to repair demyelinated areas within the CNS. It does a pretty good job sometimes and your symptoms get better - maybe even seem to go away entirely. Then....
the weather heats up or
you try to fit too much activity into a day or
you get run down or
menses has your hormone levels askew or
maybe there isn't anything you're even aware of AND
BAM!
Signals falter and symptoms reappear for various lengths of time.
The repair job can't handle the load because as good as it is it can never equal the perfection of the original (unbroken) connection that handled almost any variety that came along.
It is certainly possible to have repeat episodes of ON. It is also quite common for ON to take many, many months to stabilize at what will become the final 'new norm'. Until it does I'm thinking it will be at least as susceptible as other body parts to symptom flux. And remember, (unfortunately) it will likely never be quite as good as it was before. Maybe that is what your doctor was trying to remind you of.
When a previously existing symptom occurs with increased and lasting intensity or extends to a new area your doc may decide you are indeed having a true flare - an exacerbation - an extension of damage - or a new episode of ON. This does not mean it is worse now than it was yesterday or last week. This would be a new all-time record for intensity or area of involvement.
Flares that are treated as such but appear to be near identical repeats of past flares usually have long stretches of symptom free or near symptom free time between them (think months to years).
There is also something called paroxysmal symptoms that can occur with MS. Symptoms in this group are usually very brief in duration but can come and go in bursts over the course of several hours or days. They are associated with mechanisms of nerve conduction called ectopic and ephaptic transmission. (There are HealthPage topics on this as well as more in-depth information about the different types of exacerbations. The index of HealthPages can be found at
http://www.medhelp.org/health_pages/list?cid=36)
I hope something in all this helps. As usual I got very carried away so hope I didn’t just end up adding to the confusion or end up boring you to death before you got this far down the page.
As I started out saying, this is a hard thing to understand (and I’m sure I don’t have some of it quite right yet either). Keep asking questions here and of your doctor and TRAC your individual symptoms so you will learn how and when your body is apt to act in different situations. In the end it may be the only way to learn when it’s safe to handle symptoms on your own and when it’s best to call in the professionals.