USED! and angry about it in a surprisingly calm way

Here's the story. I can rant on here because I think I only gave this blog to one person here in town and that was a long time ago.
There is a little girl (I'll call her J) at that comes to church sometimes with her mom and the girls just love her. J is a year or two older than Madelyn. She is really sweet and I do enjoy having her over. She is also in Brownies with the girls. Every other week lately with Brownies, the girls want J to come home with us. We did that a few times last year and last fall, but since I am normally working those nights lately, she can't come over.
Last Saturday, the girls were going to be on the Cookie Booth from 3-4:30, so I told them that I would call J's mom to see if she would drop her off at the Cookie Booth and I would take her home with us when I picked up our girls. She said she would do it. She has told me numerous times to keep her as long as I want, come get her whenever I want.
Well, to the girls' disappointment, she didn't show on Saturday.
When I dropped the girls off at Brownies yesterday, the leader came out and said that "J's" Mom had called her today and told her that I was picking "J" up and I was going to take her home and feed her supper.
I said, "no, that agreement was for Saturday, not today, I am working tonight" So, in the interest of her child, seeing how I care more about her whereabouts than she does, I told the leader I would take her but I would have to get ahold of her Mom before 6:30 when I left for work. I was not leaving her with Aaron and the kids all night.
Leader gave me J's mom's cell and home phone. When I came back an hour late to pick all the kids up, I sat in the parking lot first and tried to call the cell - no answer and the mailbox was full.
When I tried to call the home phone, I was able to leave her a message telling her that our agreement was for Saturday and that I had to work tonight, to please call me at this number and that I had to have her somewhere by 6:30 when I left for work.
I ended up calling into work, because I wasn't feeling well, thank goodness, because I never heard from good ole mom.
Giving her the benefit of the doubt, I waited. Finally, I called the home phone (where I left the message before) and immediately got an answer.
Here is how the phone conversation went.
Them "hello"
Me "hi, can I speak to _______?"
Them "who is calling?"
Me "Karen Pirkle"
Them" Mom, Ms. Pirkle is on the phone"
What I hear in the background...
J's Mom "why did you answer the phone? "
J's Sister "I don't know, I didn't let it ring long enough for the ID to show up"
I sweetly talked to the smooth as honey voice on the other line, trying to process what I just heard. She told me they were home now (like they had just walked in the door) and that I could bring her home whenever, but feel free to keep her for as long as I want.
In other words, she had long gotten my message saying I needed to leave for work at 6:30. Not only did she expect me to take care of her daughter for her, but could care less about my job, and I had to drive the 10 miles there and 10 miles back to take her home.
It was 6:30 when I called and she wasn't planning on answering the phone for a long time apparently, if at all. I wouldn't be surprised if she would have never answered the phone and let J spend the night.
I told Aaron that I hated to even take her home to a house that didn't want her around. When we dropped her off there were about 5 cars in the driveway, a man outside leaning on a car smoking, so we waited until the door was open and she disappeared inside before we left.
I think there will be a confrontation at some point. I will keep the blog updated whenever it happens. Just had to vent now though.
Any suggestions?