I get nightmares if I drink milk before bed ... ?

If I drink milk even 3 hours before I go to bed, I'll have nightmares. I don't know why this is. I know that milk makes you sleepy, and I often will fall asleep after I drink a glass of milk if I'm just laying around. But each time I do, I have nightmares. I have tried different things that I've eaten before bed, and I still only get nightmares after I drink milk. Can anyone explain why to me ? I'm so confused !