My Daughter (4 Just A Few Weeks Away From Being

My daughter (4 just a few weeks away from being 5) went to bed and was just fine. About two hours later she started screaming and crying like she was in pain for a brief second then stopped. We rushed to her but she was completely lethargic. Nothing was waking her at all no matter what we did. Her body was fully limp. She acted like she was dead, and did not respond to anything no matter what. We called 911, doctor at the ER said she was perfectly fine and nothing was wrong, he couldn't find the cause. I'm looking for a second opinion because he said "don't worry about it at all"