I am having the same problem. I got diagnosed with retrocalcaneal bursitis.

I would go to a podiatrist for a proper diagnosis. You can have bone spurs that cause the issue. It is helpful to know if it's that or simply overtraining or bad shoes is causing the issue. They x-rayed me in the office - it was only one visit.

My podiatrist recommended taping the bursitis, but I find that just compresses the bursa and doesn't help with discomfort. So I took a week off and then cut the back out of the heel of my shoe (not my pic, it looks like this). I also only wear flip flops when I'm not running. It has been three weeks since then and my bursa is shrinking and I am completely pain free.

I have heard icing the bursa helps, but I haven't try that. PLEASE write back and let us know what you did and how it went.