Lyme disease can affect many different body systems, and can manifest in any of dozens of different symptoms. often it affects a person's balance. often it causes great pain–sometimes in the joints, sometimes in the muscles, often in the back and neck. It can affect breathing and circulation. It can cause numbing of the limbs. All of these things could affect a person's ability to walk.
It's important to learn as much as you can about this disease, and be evaluated by someone who really understands it. (Sadly, most doctors don't.) You need to be seen by a doctor affiliated with ILADS. (see link below.)
Good sources of info about Lyme disease:
lymedisease.org
canlyme.com
lymenet.org
lymeinfo.net
lymediseaseassociation.org
ilads.org
betterhealthguy.com