You were very clear about when you were available to do the trip and she basically said bump that. I'm staying an extra day. If yall were organizing independently and planned to just meet up then cool. But it's a bit disrespectful to not even ask if it's ok to extend an extra day especially when someone else is funding the trip for you.
However there's very little reason things should've got to this point. I don't see why you would offer to take a "side chick" on a weekend getaway for her bday like thats your main. This is where lines get blurred and confusion comes in. If you someone I just been fukkin on some FWB shyt then I might take you out for dinner or something. But a whole trip out of the country? Nah that's relationship behavior.
I'm gonna give you benefit of the doubt that you been handling your business in the sheets so if she getting her nut and you getting yours then I don't get the whole "it's the least I could do since I been fukkin her all Summer" as of she did you a favor.
TL:DR stop with the blurred lines. If you simply want an escort to accompany you on a trip then establish that from jump so she knows exactly what time it is.
you dont understand the scenario breh lol. Lines were not blurred, chick was giving him a shyt test to see if he was a simp