Is she planning to cheat? I'll try to summarize quickly. Been dating for 5 months, her birthday is coming and planning to celebrate with vacation. One week ago, here's the summary conversation when I picked her up:
Her: So this creepy resident physician at the hospital (she works as an RN) asked for my number and I had to give it to him cause it was so awkward. He's been texting me asking to hangout.
Me: Sounds like a cool guy, when's your date?
Her: Next week...just kidding. Does it bother you if I hang out with other guys?
Me: Nah (I changed the subject at this point)
She jokes and is sarcastic, so I assumed this wasn't anything serious. However, she seemed to act different that night and it was first time since we've been together that we didn't have sex. Even though, I tried to initiate several times.
That night I checked her phone (I know shouldn't have; I debated for a while but finally did), I found the conversation between her and the physician. The texts ended with her saying "she might be free next week..."
I tried to focus on other things, however today we had a FB Conversation:
(Joking mostly entire conversation)
Me: Well than I'm gonna file for divorce and take your goldfish :p
Her: Fine with me I have a date this week 
(however at this point I started thinking about the text again)
Me: Do you really? Thought your seeing me this Thursday?
Her: Yup and might have cancel on you lol
Her: (few minutes later) jk I don't go on dates what time works for you Thursday?
The issue: Her birthday and the fact that I mentioned that I would pay for the vacation (she got me something expensive for mine, so it only seemed fair). I'll have to book it by next week, but I don't feel like spending $1,500 on her after seeing that text message. In addition, If it's not this guy now, it will probably be someone else after the vacation.
However, I'm also confused about why she would mention the date in a joking matter twice now? It's like she wants me to talk about it? Should I have simply said that it does bother me? I guess I will see how she acts this Thursday? Should I bring the date issue up and talk with her about it?