Sometimes all you need is a line for ending the conversation. Saying something like, "Hey, it was great talking to you, and I've gotta get to work. I'll talk to you later, eh?" Of course, change it around to be less Canadian.
Or something like, "I've gotta go now, I'll talk to you later" can be quite enough and she won't take it negatively (unless you keep doing it and don't tell her that you're actually getting busier).
The others have offered great advice. Talk to her about it. She may be upset but there are ups and downs in relationships. especially pay attention to what Dating Specialist said.
Good luck!