is he cheating or what?

Sooooo me and my boyfriend walks into his room and I find a 36A bra on his bed... Now my boobs and his mother's (who we live with) are way bigger than an A & I'm sitting here confused on how I should react... He told me he doesn't know how it got there... About a week or two ago I was away at college so I really don't know what was going on.