I don't know what happened the first time. He wouldn't say. At the time, Chuck lived in the House with us. Castiel showed up with a cut on his neck, it was very serious. Chuck called me to patch him up, and I took care of him while he was healing. Apparently, you, the version of you which was here, had been trying to drink his blood. Trying to treat the withdrawal he was brought here in. 'Hopped up on demon blood' is what I was told. Castiel feared the withdrawal process would kill him, and was attempting to find a way to save him.

[That was a lot to take in, especially if the Core had brought Sam from a previous time point.]