The anatomy of the inside of your ankle is your tibia (which is the prominent bit of bone), you have a ligament called the deltod ligament (which resists eversion) and then the tendons that invert the foot.

To strain your deltoid ligament is actually quite hard and you would have twisted your ankle into eversion to do this. This is unlikely then as there was no incident.

What would be interesting to check out is your tibialis posterior muscle. This cups around the posterior and inferior aspects of the medial malleolus (the prominent bit of bone) and its job is to maintain the medial arch of the foot. If there is an injury too it you get a gradual collapsing of the arch which also then puts it on stretch.

As you look at your feet, stood normally, is one arch different to the other? Then sit down with you legs out straight and the insides of your feet together. Push them against each other - any pain? Were the orthotics you were given custom made or just off the shelf?

What I would try is ice to the tendon to decrease any inflammation. Then what I do is use a low-dye taping technique, which basically holds the arch in the foot. If that decreases the pain it confirms my diagnosis for me. From there I would either stregthen the muscle to amintain the arch itself or have custom made orthotics for the patient. Which one of these I do depends on the severity of the arch collapse.

Hope this explains things a bit. Look forward to the answers to the questions