Why was he uncomfortable talking about his girlfriend?

I was speaking to one of my guy friends from work. I'm 17 nearly 18 and he is 23. We were on a break together and just talking about random things. He mentioned he was going to Spain and couldn't wait to relax by the pool. I asked him who he was going with then he seemed uncomfortable,broke eye contact and said, "Uhhh,just my girlfriend." Then he changed subject. I don't understand why he felt uncomfortable. It's not like I was asking for details about ANYTHING. He's only ever talked to me about her once before which was a few months ago. They live in different towns so he doesn't see her much and I've never met her.