What could explain the cramp in my foot?

Flatfoot: You could be stressing your arch and the intrinsic muscles can be fatiguing from your arch collapsing. Also, dehydration or electrolyte imbalance can be a cause. Start with drinking lots of fluids and maybe an over the counter arch support.

Dehydration. : There are several ways to treat night cramps, including drinking enough water during the day, making sure you're eating a balanced diet with a multivitamin, and stretching exercises. If the cramps occur during the day, you may need different shoes, or a custom orthotic. If the muscle spasms continue, your doctor may prescribe you a medication to help.