Why do i get sore middle bottom of feet sometimes?

Sounds like : Plantar fasciitis which is a strained arch. Usually the result of prolonged weightbearing activity without adequate foot support. More supportive shoes, inserts, stretching, ice and massage may help. If still a problem, see podiatrist.

Plantar fascitiis: Is the likely problem. This is a broad banded ligament that runs under the foot. It can get inflammed or degenerated, pain often on first few steps or after periods of rest when the person goes to stand again. If that sounds like it. Try wearing arch supports, icing it, taking antiinflammatories and if pain persists see a foot doc.