I suddenly for no apparent reason have a sharp pain at the back of my ankle just above the heel. do i need to see a doctor? how should i treat it?

Depends: If it is fairly mild, trying some ice, an anti/inflammatory, and some rest would be reasonable. If it is severe, I would probably be seen. We will occasionally see a spontaneous rupture of the Achilles' tendon, for example, that can cause severe pain.

Tendonitis: Sounds like Achilles tendonitis, bursitis, or paratenonitis. Most likely tendonitis. Try Rest, Ice, mild compression (ace wrap), and take a nonsteriodal over the counter antinflammatory medication for a few days. Also don't wear flat shoes or go barefoot. If it doesn't clear up in a couple of weeks it's time for an office visit. Good luck!