Why did my guy friend start acting so cold?

There's a guy that I have to work closely with and over the last several months we have had a workplace friendship. (I am already in a relationship which he knew about from the very beginning). He was always so nice to me and we enjoyed working with each other. I went on vacation with my fiancé and leading up to that he became a bit cold towards me. I didn't think much of it, as he could've been having some issues in his life or whatever. I return from my vacation and his demeanor is very icy towards me. He won't look me in the eye, won't speak to me, and acts like we were never work friends.
I can see that he is his usual friendly self with others though and can't see any changes in those interactions. He acts bizarre with me and it's just a tense situation.
Why did my guy friend start acting so cold?
Add Opinion