What Causes Burning Sensation Under The Foot While Suffering From Foot Sprain?

i twisted my foot 4 days ago and had x-rays at the er, they said that i sprained te ligaments. i have appt with an ortopedic next week. tonight i used, for the 1st time, and knee walker for about 5 minutes. wen i laid down i experiended a burning sensation in the bottom of my foot. can you tell me what caused this? i should probably also tell you that i have p.a.d.