Why am I dying when I go outside?

It's most likely that your character is a vampire. There's a quest you can complete, found on the wiki, that will cure this. It's also possible to cure it using the console. Other than that, you can avoid damage by staying out of the sun, and fast traveling at night.

You have vampirism.

An easy way to get rid of it: Theres a glitch. First go into the water then drown. After you're dead, load your last save and vampirism will vanish, but not permanently. Maybe the next month you'll get vampireism.