Sounds like possible heat stroke/exhaustion . I know that some doctors don't like for you to "self diagnose" but it's worth mentioning. ERs are for emergencies and although a heat stroke is a true emergency, you waking up and passing out wouldn't give them the idea that it was heat stroke/exhaustion unless you forfeited the happenings of the prior day. Their job is to make sure you're not actively dying and you weren't, that time passed and you were stable other than probably being very dehydrated at that point. Dehydration usually accompanies any heat injury. It's important to remember that doctors aren't psychics and need as much help and info as possible to diagnose correctly. Make sure you're passing that info on so they know where to look and what tests to do. Good luck!
If they can't find anything wrong, perhaps it is in your imagination from over-examining your body. It could also be panic attacks causing you to feel weak when you really are as strong as before.