Throwaway account. 
As the title says this morning I (22m) woke up at my partners 23(nonbinary) apartment. I was late to work already so I was rushing to get my stuff to shower. As I’m grabbing some of my boxers I notice a pair of boxers that are clearly not mine they’re neon green something id never wear. I ask them who’s they are and they said they thought they might’ve been mine ( but admitted to  never seeing me in them) so they put it in the drawer where my clothes are kept. After I said they’re not mine, They said the might belong to their sister because she wears boxers. I told them to ask and they winded up  not  belonging to their sister or their cousin who occasionally stays over. We both leave to work and I’m obviously upset. At work they send me a message saying that the boxers belong to their friend. I attached the image below. In it, it seem likes they’re almost trying to get them to admit that they belong to their friend. I’m still very skeptical whether I should believe them or not. We spend a lot of our time together and have a very healthy relationship I wouldn’t think of them as a cheater but I’m don’t know what to think and as much as I want to stay together I feel like the doubt will just eat me up in the future if we continue the relationship. I’m conflicted but feel like we should split. Am I overreacting for thinking that?

AIO Boxers found at my partners house I feel like they’re cheating