You are proficient with medium armor.
Armor Proficiency (Light)
A character who is wearing armor with which they are not proficient applies their armor check penalty to attack rolls and to all skill checks that involve moving, including ride.
When you wear a type of armor with which you are proficient, the armor check penalty for that armor applies only to Balance, Climb, Escape Artist, Hide, Move Silently, Pick Pocket, and Tumble skill checks.
Barbarians, Bards, Clerics, Druids, Fighters, and Paladins automatically have Armor Proficiency (Medium) as a bonus feat. They need not select it.