You and Alex were looking around in an abandoned building to try and find Sarah.. you both went upstairs and looked around.. As you two were just about to leave, (you and him were going downstairs) he hit you on the back of your head hard and you fell down the stairs. You woke up later with a concussion and at his house.. you about to ask him what happened but he spoke.

"you fell down the stairs."

Alex lied. but you didnt know that.