This is my experience of ankle issues, reader beware and all.
The area you’re describing sounds like your achilles tendon where it attaches to heel.
Overuse/over training is a common cause. This could be compounded with uncontrolled movement of the ankle, weak join/tendons, incorrect tracking of the joint/tendon.
I’m currently recovering from achilles tendinopathy myself and getting treatment and advise for a great physo. If it is the achilles then it can take a long time to put right. I’m currently 2 month in and nearly ‘fixed’ some people suffer for years.
What you must NOT do with tendons is push thought the pain as it’s possible to damage them to a point it’s irreversible (you change the structure of the fibres of the tendon).
I had some achilles tendinitis (now called achilles tendinopathy) went for a 40 mile ride (mid length ride by my standard) came home and it had a lump (size of a split pea) and made a creaking noise when I moved it, not good. Fortunately for me it’s still at the fixable end.
My recovery/strategy is covered in 3 broad strokes.
1) Shut the tendon down. You need to allow the tendon to recover, for the inflammation to settle. So rest it, don’t ride, and try to keep off it. Ice it. Get on the Ibuprofen.
2) Once the inflammation has settled you can do exercises to help/speed the recovery. The number one thing is “achilles eccentric loading exercises”, Google it. Stretching will help (but we all do that anyway!). Use a foam roller to loose the calf.
3) Fine the cause and fix it!!
Overuse is a common one, so built up slowly.
Tracking is also a common one. Do you your feet roll in? Does you knee track you foot/ankle? Do your toes point towards/away form the bike? Are you pedalling with the tips of your toes? The list is endless as suggested a bike fit will tell you this. Rule of thumb is the tendon kept straight when you pedal. If you put an S shape in then that’s not good for it.
My issue have been traced to weak ankles brought on by overuse (dame you strava may massive). My muscles that support the ankle are not strong enough to support the achilles tendon which in turn forces load on the tendon through uncontrolled movement. 5 days cycling on the bounce on a Northern Ireland tour caused the initial achilles tendinopathy to flair up. I then pushed it further. A easy test to identify this weakness:-. Stand on one leg bend your knees. If you wobble around loads you have weak ankles. The solution is simple, do loads of balancing work. I now have a balance board which I use while watching the TV.
As I said before this is my own experience and everyone is different.
I would never tell people not to see the GP but 99% of the time the GP will send you away with a perception for anti inflammatory which the pharmacist will suggest you just buy over the counter (as it’s cheaper than paying the perception) Ibuprofen. Ibuprofen and rest will probably fix the symptom but will not find the cause. Costly but I recommend getting this checked out buy a Physio.
I’ve done a lot of research on this and “achilles eccentric loading exercises” seems to be the most affective treatment.