In my experience, this can be one of two things:
1. Low interest and/or a needy woman looking for you to beg to get extra validation from you. Translation to man-speak would be, "Hey, I know we have plans together tonight, but my friend invited me over and I'm really not sure if I'd rather see her or you, so why don't you tell me how much you'd love to see me so that I can consider that as input for whatever my final decision ends up being." I mean, WTF does she honestly expect you to say in response to that? "No baby! I want to see you! Please! Please see me instead of your friend! I miss you!"
2. This can be part of a woman's attempt to feel your masculine dominance. You have to try not to let it bother you because it is just in a woman's nature to want to feel that part of you, and when she craves it, she will throw you the ball in hopes you take the hint and give her the dominance she wants. So usually when I get this kind of line, I will just say, "You and I had plans. You can see your friend another time." Many women truly love being told what to do (I know this sounds chauvinistic but it is the honest truth whether most women would want to admit it or not) and if you have any experience with **** (in real life or through reading), that's a huge part of it. It's not all chains and whips - it's about dominance and submission in all aspects.
The way to determine whether it is #1 or #2 is partly based on your perception of her interest level in you, and then to assume it is #2 and respond in the way I mentioned and stand your ground. Even if it is #2, she might push back on your dominance but hold firm. In the end, she will either submit, or not, and if she ultimately does not, then the situation is #1 and you know you should next her. Just don't let her try to get you to beg or say how much you want to see her during this exchange - she will likely try. Keep the dominance frame. For example, if she says, "Well I kind of want to see my friend - Do you really want to see me that bad?" You just say, "I'll see you at 8 as we planned."
Unfortunately, if her reason was #2, your response didn't really help you figure it out or give her what she needed, and her response to you could very well mean, "Hey, I was hoping you'd be dominant and tell me what to do but you just 86'd me! Why would you do that?" I've made this mistake in the past... one that particularly comes to mind happened a few years ago. A girl tried to flake on me a second time (to hang out with her girlfriend) and I didn't even respond to her. She really wanted me to tell her "no - you will make good on our plans. You can see your friend another time when we don't have plans," but I didn't. Even so, when I didn't respond, she texted me again later saying, "well, let me know if that's ok" - again, she wanted me to tell her NO it isn't - to command her. She was literally asking me for permission and guidance. I ignored again and an hour later she texted, "ok I'm going to ditch my friend so I can meet you as-planned." I was upset and didn't respond then either because I was done with her but you can just see from this whole exchange, all she wanted the whole time was for me to tell her what to do. She even told me on our first date that she was very submissive and liked to be dominated, and how much of a turn-on it was to just be able to turn her mind off and let someone else think for her and command her and tell her what to do and completely submit. Often these women are the LEAST submissive outside the bedroom and CONSTANTLY challenge you..... and now you know why: They crave dominance and the button they push to receive it is being bratty. I now always respond to bratty behavior with extra dominance and control and it fvcking works like magic. If it doesn't work, then you know you have a low quality woman with some form of personality disorder, or a woman who isn't interested in you. In my experience, once I opened my eyes to all this, I have found that probably 90% of women want this kind of dominance. Blue-pill brain washing tells us it is unhealthy and wrong for a man to do this but that's just more examples of how third wave feminism is screwing things up for women, and men. They don't know what they want and if you let them try to make the rules, they come up with things that run contrary to what they crave. For anyone who reads this and thinks it is subversive or chauvinistic - all I ask is that you be open minded enough to TRY it and see the outcome.