Why is he mad??

Me and this coworker became really good friends quick. Keep in mind as you read i told him I was currently seeing someone. We would always go for walks together on break. We would always JOKE around with each other and come up with our future kids together names. And what color our wedding was going to be (obviously a joke). I told him I was going out with my bf Friday night. I missed Friday off of work but didn’t even see the guy lol different issue. But when I came back to work today he acted all mad & wouldn’t talk to me. He took a walk by himself. What do you think his problem is???