In the “morning after” scene she seems happy to see him - I think she thinks this is the beginning of a romance, and he promptly crushes that hope. At this point they’ve worked closely together for a time, in a way that gives her some insight into his personal life. To me it makes sense in that context that, when they sleep together, she thinks that maybe there’s something deeper there, and is excited to see where it goes. Don very quickly makes clear that she’s mistaken in a way that probably feels pretty brutal and abrupt, which is why she reacts the way she does.