This is a possibility for the note that Spike belched when he was with the other dragons. Since it was thrown into the lava before he could read it, We'll never be sure. So, here it is.
Dear Spike,
It has come to my attention that you are trying to find out who you really are.
Spike, you were found as an egg in the Crystal Caves. The pony who found you waited for seven days and nights. Your parents never returned. It's believed that you were abandoned.
Twilight hatched you using her magic, there for, according to the creature laws, she is your mother.
I'm sorry to have to drop this on you. I hope you're alright with this.
Yours truly,
Princess Celestia.
Like I said, it's a rough cut. But think about it. It could be what the note said. We'll never know.