If you read that creepypasta thing about Lavendar Town syndrome, then it's probably a sign that you believe it. If you believe it, you will experience that stuff. The story was made to mess with people's heads. But the story's fake, not real, so don't worry about it. Also, you said this occurred in LeafGreen. Well the creepypasta story applied only to the original Japanese Red and Green for Gameboy, not FireRed/LeafGreen. So either way you shouldn't worry about it because that story's not real, and even if it were it didn't apply to FireRed/LeafGreen.
me, I'm the Pokemon Guru, listened to Lavendar Town song many times and nothing happened