You were coming into the bunker late. You didn't let anyone know that you were coming back, so when you came back making noise, it woke Dean up.

Dean snuck up on you and pinned you down to the ground, causing you to hit your head as you were pinned. Dean heard you groan, and he quickly got up. He didn't know that it was you and he felt terrible.

"Oh, God. I'm so sorry, I had no idea it was you. Are you okay? Talk to me."