Many possible issues. The answer may be as simple as tired feet, electrolyte imbalance, circulation issues, or neurological deficits. See your local podiatrist for accurate diagnosis and treatment.
Many... There can be many causes of this, but a common one is low mineral levels. Magnesium and potassium are the most common, but i suggest a nutritional evaluation to make sure. In the meantime, adding some magnesium and potassium supplements should be ok as long as you don't have any other conditions.
Foot strain. Could be from foot strain without proper support. Try an orthotic to give you support.