I have worked on a couple male clients in their 50's that have had this happen. It is, like Hans mentioned in his post, a possible partial tear or "fraying" of the Achilles Tendon. It can be quite painful and can take quite some time to heal. It has a lot to do with overly tight posterior leg muscles and really does need to be looked at by a physician for proper diagnosis.
Both of my guys saw podiatrists and were told to wear a small heel lift to take some of the tension off of the tendon during the day. My husband went through this also. I literally could palpate slowly and feel the frayed edges, I worked on it, it but in truth it still took over 6 months for him to really recover. What helped the most is the wearing of one of those "boots" that podiatirists prescribe for plantar fasciitis. It keeps the foot in dorsi flexion while asleep and helps the tendon heal in a stretched position.
Working the entire posterior leg seems to really help loosen and lengthen the hamstrings, gastrocs and achilles tendon. I usually could find and release small trigger points in between the 2 heads of the gastrocs which would help relieve their tightness and symptoms while healing.
In all 3 cases that I saw and worked on it still took over 6 months for recovery. All three were prescribed anti inflammatories by their Doctors.