What can cause a tender heel?

Plantar Fasciatis: Plantar fasciatis is the most common cause. I suggest you see a podiatrist.

Many things: Most commonly heel spur syndrome/plantar fasciitis. Could also be a stress fracture or nerve entrapment or trauma or an arthritity. Could even be a corn, wart or foreign body such as splinter or glass. If it persists see a podiatrist. Examine area, try supportive shoes.