My boyfriend gave me his fb password then recently changed it so I can't log onto it?

This may sound so childish but I dont care. He gave me his password in the past because he had cheated on me, and he gave that to me to prove he had nothing to hide. Recently, he changed it and is reluctant to tell me the password. For some reason this is putting our relationship on the line, and i feel like its for a really silly reason but I just can't drop it. Why suddenly not trust me to have it? I feel like he's hiding something and its driving me crazy. I haven't done anything to not deserve to know it anymore, and he knows mine. I don't know what to do?
My boyfriend gave me his fb password then recently changed it so I can't log onto it?
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