Contrary to popular belief, not all heel pain is plantar fasciitis. The plantar fascia is a very specific structure in the foot and is made up of three parts: the medial, central and lateral bands.
The pain typically occurs first thing in the morning on your first steps when you get out of bed; after extended periods of walking and when getting up from having been sat down.
The pain can only be under / on the very bottom-side of the heel AND/OR within the arch. Pain elsewhere in the heel foot, including on the back of the heel, is unlikely to be plantar fasciitis, and treating htis pain the same way as you would treat plantar fasciitis may make your pain worse.
Plantar fasciitis is one type of heel pain, and is one of the most common foot pain conditions that I see.