What could cause morning heel pain?

Heel spur/fasciitis: Plantar fasciitis with or without heel spur is classicallt very painful upon first few steps after rest before leveling off a bit with continued activity.

See below: Plantar fasciitis or a heel spur or bone bruise of the heel, nerve injury.

Plantar Fasciatis: Is the most common cause of pain after rest.

Many causes: Heel pain is most often caused by plantar fasciitis, a condition that is sometimes also called heel spur syndrome when a spur is present. Heel pain may also be due to other causes, such as infection, tendonitis, arthritis, stress fracture, nerve irritation, retained foreign body, bone tumor, or a cyst. Have it evaluated by a professional and get the appropriate treatment.