I am having charlie horse or swimmer's cramps in my feet, frequently. What could be causing these?

Vitamin deficiency. Is a possibility. Potassium or magnesium would be the ones i would think of first. Your intrinsic muscles might be overworked as well and sometimes an orthotic can be helpful for this. Finally, make sure you are well hydrated.
Electrolyte . Electrolyte imbalance is one likely cause. If you are active then your body is being deprive of electrolytes and they need to be replace. Gatorate helps. If you smoke, or have vascular issue this may be caused by clogged arteries. Please go and be evaluated by a podiatrist or a vascular doctor.