I feel like I have been living with a stranger

Last night I stumbled upon some pictures of my boyfriend and girl and they were holding and kissing. I realized this had taken place while we were together. He went to Thailand for 3 weeks and from what I can tell had a relationship with this girl. All of this happened over a year ago. Its not like it was just a drunken stupor never gonna do that again type of thing they hung out for weeks and he never mentioned it. When I asked him about last night he knew he was cornered.