Why did my heel get swollen and painful overnight? It's really tender; it hurts to the touch.
I am not a doctor and can't diagnose you over the internet, so you're going to have to see a doctor to get this figured out if it doesn't go away. Achilles tendonitis can cause pain and swelling in the heel. In Achilles tendonitis the tendon attached to the heel from the Achilles tendon becomes inflamed. This type of injury is usually exacerbated after exercise.
A trapped nerve due to a sprain or trauma can cause swelling and pain; although unless you kicked something unconsciously when asleep, you would probably have noticed something.
I'm not sure if plantar fasciitis can cause swelling, but it often causes pain in the early morning, which often gets better once you start walking on it.
These are just some ideas; only a medical professional can diagnose you. Good luck!
Here are some links that might help:
Foot Pain – An Introduction
What Your Feet Say About Your Health
Foot Pain - Forefoot Pain
Hall of Shame: Shoes That Cause Pain