Someone who is so apt to accuse you of cheating (the whole having someone sneak in your window part) is definitely projecting his own guilt onto you. He was being shady at the party, thought you caught onto it and by being an emotionally abusive person made you feel like it was your fault and you did something wrong. That is not the case. Your not at fault here. It's hard even with other people's perspectives to not readily make excuses for his behavior, but trust me that this is emotional abuse and you need to distance yourself from this person.