So, what Alexi did was basically let himself feel Erik's emotions towards him? The pain is the pain of his hatred, humiliation, and anger at what Alexi has done to him. It doesn't really have any correlation with the pain from his bloodlust, right? 

I R so confused... Erik is even more confuzzled. He ... didn't even know the potion could work on humans, and he's never heard of it causing pain. Actually, I thought it just caused pain from people refusing to take it, and that once they did they were fine? Was I totally wrong about that?