I found another girl's underwear in my boyfriends room?

Mintyboi
My boyfriend lives in a shared student accommodation. I finished work and then went to his. He did some of my washing for me and in the pile of clothes I found another girl's underwear :( I asked him about it and he got very defensive saying that he would never do something like that. And how they must of been left in the machine from before.
I want to believe him, but part of me doesn't
I found another girl's underwear in my boyfriends room?
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