Unplanned sighting

I'm at the mall/grocery store/on campus/anywhere and I see an acquaintance. What do I do? I hide. Every time. Even if they are standing in the store/aisle/entrance to a building/etc that I need to go through, I will turn the other way and go to a different store/aisle/entrance. I don't know why this stresses me out so much, but it does. It's especially bad when I see girls that I recognize from one of my classes in the restroom while washing my hands. Awkward.