I've been mostly wearing heels the last few 5 6 months. i wore flat shoes a couple of days ago and it hurt after a walking/standing. whats wrong?

Changed the stress: Wearing heels over a prolonged period of time creates a muscle/tendon memory pattern- meaning these soft tissues are used to being in a certain position. Then, when you change to flats, you have put these tissues in a new and uncomfortable position. One example is the Achilles tendon area. When wearing heels, you take tension off the tendon/calf muscles, and then going to flats stretches these.

Where is the pain: Heel, Achilles tendon? Sometimes a flat shoe can over stretch and aggrevate the Achilles or heel ligament if it has been used to a more contracted position (wearing heels)... Need more info to really answer the question though.