I am 40 years of age, Male, 5’8” 140lb, White British, painful on & off for a few months, I am fit & well but do have hypermobility

I keep experiencing pain in the lower part of my left leg. It’s about 3 inches up from the bottom of my foot & at the back to the left slightly so to the outside of my leg. I wouldn’t say it’s the angle joint itself. It feels deeper than the Achilles tendon. It’s painful when I point my foot to the sky & to the side so on dorsiflexion & eversion. I have had an X-ray & all seems fine. Sometimes it just disappears overnight or through the day. Sometimes if I put my foot into a certain position it immediately disappears as if something has just snapped into place. I have very flat feet which I don’t think is helping. The first time it happened was when I was cleaning the car. I had my foot out of the car door pressed on the floor & I tried pushing myself further into the car & my foot slipped on the wet ground. It has never been the same since. It’s painful walking up the steps too & preventing me doing my normal routine at the gym.

Any ideas?