I have pain at the back of my heel, little above the arch. And I have pain in the morning. Especially the first few steps. Is it Achilles tendonitis?

Plantar Fasciitis. Pain on the bottom of the heel near the arch which hurts in the morning is the classic symptom of plantar fasciitis, which differs from achilles tendonitis. There are many excellent treatments for this and I recommend you see a podiatrist for evaluation and treatment.
Not clear. By ur description of Sx, pain in am and just behind the arch, it sounds more like a case of plantar fasciitis. It really depends on whether the pain is on the bottom of the foot or rather on the top of the heel bone where the Achilles attaches. People with PF (very common) tend to have am pain as all nite long the plantar fascia on the bottom of the foot shortens while the foot is plantar flexed.