Some people cannot convert cyanocobalamin from the injections to active methylcobalamin in the body. This is due to a genetic failure to methylate both B12 and folic acid.

The cyano will show up in the blood from the injection, but the tissues cannot use it.

You can try oral B12... use methylcobalamin 5mg daily on an empty stomach for 3 months. If there are improvements, then the B12 was your problem all along, even with the injections.

The genetic flaw is called MTHFR mutation, and can be tested for.

It is pretty common and many people have it.

Also there are two other blood tests... MMA and homocysteine.
These actually show if the B12 is working in the body. If elevated then it is most likely your B12 treatment isn't working for you. Doctors who are not keeping up with B12 treatment follow the old saw of one injection/month...and this does not work for everyone.

You can buy Methylcobalamin inexpensively at iherb.com, swanson's, or Puritan's Pride. Not commonly in stores yet, you may find it at a big health food store, but it will most likely cost more $$ than online.
example:
http://www.puritan.com/vitamin-b-12-...000-mcg-032860
It is important...must be METHYLB12

Also come UTI treatments use drugs that can cause PN.
Macrobid (Macrodantin)
Cipro, Levaquin, Avelox or other fluoroquinolone.

Do you know what antibiotics you took and how many times?