Regular doctors do not interpret vitamin testing accurately. That is just the way it is.

Labs in US still report levels of B12 at 250 as "normal", when new guidelines for doctors are in place which start at 400.

What I post here is also at this medical website:
http://www.aafp.org/afp/2003/0301/p979.html

You are only 18 yrs old. Doctors don't look for dunning things at your age.

PN is not common in your age group. Unless you have CMT that you inherited.

All PNers benefit from looking at lifestyle factors, and making adjustments based on them. For your age, the largest potential, is vaccine injury, and/or gluten intolerance. Next comes mechanical compressions to your spine from the running activity.
Next is low Vit D, which impacts calcium metabolism, then comes low blood sugar (since your symptoms are worse after running).

The fact you have few periods, suggests low hormone status, which then suggests calcium problems. It is your choice to follow up on these potentials. Unless you go to a holistic doctor and have another type of thorough check up you will not find answers completely with a neurologist IMO.

When a person gets symptoms from an activity, that is a cardinal sign IMO to follow. But not all doctors will do this. So it is up to you to do this if you wish to feel better. But be advised, running is no guarantee at all of good health. It has high impact, and stresses your body, as you are now noticing. Substitute swimming or a lower impact type activity for a while and see if or how your symptoms change.

Learning about PN and how your body works is a task you need to consider. Do a little homework each day. Our subforum has many links for you to look over.
Your issues may not even be PN, they may be symptoms of something else. Only time, and learning will point the way for that.