What is causing my foot pain after walking?

Could be many thing. Need more info on type of pain, where located, how long it last.
Foot strain. Check to make sure your shoes are giving you good support and that they are not worn out.
Many possibilities. Your pain could be as a result of many different things. A thorough history and physical exam and possibly some laboratory and radiology exams are needed to determine the exact cause. Causes of pain may include: trauma (fracture, sprain, arthritis, tight shoes), infection, metabolic conditions (gout), musculoskeletal or biomechanical abnormality and neurologic conditions (neuropathy).