Medial foot pain

I've reviewed the symptoms of tendonitis, and think I have a problem with posterial tibial tendon, but my symptoms don't exactly match the descriptions.  I have a sharp pain that radiates from the inside of my arch (where the navicular is, according to the anatomical drawings) up toward my ankle whenever I bend over (as to pick up something off the floor, or to stretch) without bending my knees.   It just started about 5 days ago, and only occurs in the left foot, and does not seem to hurt when I stand on my toes or walk up or down stairs.  However, moving my heel when my knees are bent will also trigger the pain.   I've palpated the area and it is a bit tender, but doesn't seem to be swollen much.    

I recently went back to doing yoga, and am wondering if I've pulled something even though I've been careful to not strain doing any of the positions.  I have been practicing balancing on one foot a lot, although it has not hurt at all.  

I've started to ice it at night, and am using my rubber ball to massage the area when at my desk.  Are there any rehab exercises I might do to help with this problem?

Thanks.  

-- Pam