I went running on a really hot day, and when I came home I couldn't read for about an hour. Am I ok?

Get looked at. The likelihood is that you're just dehydrated or have some mild heat exhaustion. But without specifics it's difficult if not impossible to know for sure. Probably a good idea to get checked out by your primary care provider.
Dehydration or... Probably dehydration, but there could be some other type of cause that is affecting you. I would have this checked out by a Dr. to prevent similar happenings.