Why do i get these painful muscle spasms in my foot?

Fatigue: Usually the foot begins to cramp when you have changed the way you walk either from an injury or possibly from just being more active. The muscles get fatigued and generally need more support. Try some of the off the shelf shoe inserts with plastic arch supports in them. If this does not work you may need to see a podiatrist for custom orthotics.

Different reasons: Are you flatfooted or high arched? What type of shoes do you wear? Need to check for magnesium deficiency, check potassium levels, make sure you are well hydrated. Massage and stretch the spasm to help relieve it.