What could cause pain of the left third metatarsal of my foot?

Maybe sprained: Metatarsal pain may be like turf toe, which is an sprain at the joint between the big toe and the forefoot. Cold packs can help. Tylenol (acetaminophen) or Motrin for 3 days, no weight-bearing for 3 days, taping or splinting the toe joint, wearing a non-flexible shoe, avoiding sports for a few days, etc..., all lessen the pain. If not better soon, one can see an orthopedic or podiatry doctor for an exam & x-rays.

Metatarsal: There are a number of things that can be happening from a stress fracture, to tendon sprain, to a bone bruise. Seeing a foot and ankle surgeon is helpful.