This is a fairly common cause of pain on the underside of the heel and accounts for up to 20% of all plantar heel pain.  Many symptoms of this condition are similar to Plantarfaciitis, and because of this sometimes the correct diagnosis is missed.  

Unlike Plantarfaciitis, with this condition there is not usually any pain when getting out of bed in the morning.   

Symptoms are usually worse with prolonged activity. Pain begins as an ache, progresses to a burning or abnormal sensation on the plantar heel and then eventually to numbness. There is often tenderness on the inner side of the heel bone too.

The Baxter's nerve is the 1st branch of Lateral Plantar Nerve (Inferior Calcaneal  Nerve); it can become entrapped by a heel spur, muscle hypertrophy; or excessive pronation in those with Flat Feet or fallen arches.  

It is quite common in runners due to repetitive heel impact.