I agree with Damien:  Get it checked.

However, it sounds like the Posterior Tibialis tendon to me.  Google it and look at the anatomy to see if the place you're describing is correct.  Some of the pain patterns sound like tendonitis to me.

One thing that can be done (it's of general help - maybe not direct help for the tendon) is to massage out that muscle.  Put the bad ankle over the opposite knee.  Put one thumb right behind the shin bone where the bone turns to fleshy area, down by the ankle.  PUt the other thumb on top of the first.  PUsh down reasonably hard.  While making small circles with your foot (you'll feel and see the muscle contract and relax with each rotation) slowly run your thumbs up the shin toward the knee.  You'll give that muscle a good massage.

Arch support and rest will enable the tendon to rest.  Icing should bring down the inflammation.

Jim

First, the pain pattern you describe:  The fact you massage it and can walk with no pain fits EXACTLY with tendonitis.  I've had post tib tendonitids (and other tendonitis) and what you describe is common.  The tendon becomes inflamed and kind of "sticks" to the sheat that it is in when it is not used for a period of time.  My guess is that it's painful for a while when you get up in the morning.  Getting it moving in the sheath is the painful part, but once it's moving the immediate pain subsides.  If the root cause isn't resolve you'll still be aggravating it even though there is no pain.

Second, the site you referenced goes into all sorts of different reasons why that tendon may be getting aggravated.  This is where Damien's recommendation comes in:  Go to a doctor.  They can give you ideas on what needs to be done so the tendon doesn't get aggravated.

Jim