By the way, doctor: Switching from heels to flats
Q. I know high heels are bad for the feet, but when I switched to flats, my feet started to hurt. Why?
A. Wearing heels for long periods can result in a shortening and tightening of the Achilles' tendon. When you walk in a relatively flat shoe, your foot flexes up (dorsiflexes) when your heel hits the ground and then flexes down (plantarflexes) as you toe off. If the Achilles' tendon is short and tight because of high heels, the foot loses its ability to flex up and down, so switching to a flat can cause heel and calf pain. Flats can also aggravate conditions like plantar fasciitis if they lack good arch and heel support.
If you're moving away from high heels to flats, do so gradually, wearing lower heels at first. That will give your Achilles' tendon time to relax and stretch out. You might also try doing some of the standard exercises for stretching the Achilles' tendon.