What Relieves The Pain At The Back Of The Heel?

Hi, I was playing football a couple of months ago and struck the ball hard towards the toe of my right foot i.e. caught it wrong. I got a sharp pain in the back of my heal which went away after a minute or so an jogging seemed to help. I thought it might be a slight strain or something so took a month or so off but it doesn t seem to have helped as occassionally when push off that foot hard (e.g. at volleyball) or kick the ball hard it still really hurts but goes after a minute or so. It doesn t hurt to walk or run just literally when fully straightened. Any ideas what i have done or what can help? Thanks in advance Matt