Posterior Tibial Dysfunction / Tendonitis (PTTD) or Rupture (Pain on inside of foot)
The PT tendon is the primary supinator or lifter of the inside of the foot and by firing it, you can actively control pronation. Under certain conditions, and in the presence of mal-alignment, the tendon is put under excessive strain which can cause it to become inflamed, tethered, or, in a worse case, ruptured. If one of your feet collapses inward, and the arch suddenly drops beyond your control, you have, more than likely, suffered a ruptured or tethered PT tendon. That is an acute injury and should be diagnosed and treated by a doctor. You may have to wear a special rehab boot, and in severe cases, surgery may be indicated. Subsequent to healing, you can be treated pedorthically.
$ Self Help
Of course, rest and anti-inflammatories will help, but if you have inflammation that runs down the inside of your lower leg, behind the inside ankle bone, and under the arch, see a doctor. Don’t mess around with it yourself. A doctor will probably recommend the next option.
$$ Medical Supply Store Or Sport Rehab Store
These can fit you with an over-the-counter ankle brace that will give the tendon some support. High-top boots may also help, but only in conjunction with custom orthotic devices, assuming the tendon is just inflamed.
$$$ Pedorthist or Other Foot Care Specialist
Custom orthotic devices would be indicated, and they should be designed with maximum correction, at least until the problem is resolved. If you live where the terrain is uneven, an articulated AFO (ankle foot orthoses) will stabilize the foot using the lower leg as a pillar of support.
Ready-made shoes or boots can be modified to be wider in the arch area thereby giving more support from the ground to the affected area.
$$$$ Pedorthist or Custom Shoemaker
If one of your arches has completely collapsed, and become deformed with a prominent bone protruding in the arch, custom shoes or boots would be indicated to accommodate the shape.