Advice please

I watch a toddler boy ever Tuesday and Thursday for about an hour and a half to two hours for a friend. I have been doing this for over 6 months with no compensation. Originally I agreed to this because I thought I would have physical therapy soon and would need her to watch V as well, but I don’t. I also thought this would be a one time deal of 8weeks of therapy for her and then no more, but she was sent back to therapy and no end is in sight. I have never asked her to watch V and am no feeling very resentful of her and her boy. I don’t like that I have to give up those two hours everyday to a kid V barely gets along with, both boys like the same toys and fight like mad. On top of that She just expects me to take her son for every appointment she has without even asking me about it. She also refers to me as her babysitter and I hate that because babysitters get paid and she never even treats me when we go out although she always says she will. Maybe it won’t be so bad when the weather turns really sour but right now I hate it. I want it to stop and don’t know how without ruining the friendship.
Any ideas?
Oh I have also met a new friend J. and her little girl Abby. V loves Abby and I like J. We are close in age and share a lot of the same interests, plus our husbands get along. My other friend N., whose kid I watch, also knows J and is now trying to cause problems because she thinks I prefer J to her. I know she talks behind my back to J (J told me) and has even made a suggestion that I like J in the more then friend’s kind of way. It basically feels like we are in high school. N says that I always want to hang out with J and take her only as an alternative to J. She says a lot of stuff that isn’t true and I have told her that all of this is not true but she seems to think of these friendships as competitions. I have never been more popular. Of course what gets me is that if I was sexually attracted to J, N had no business pointing it out and ruining my chances. I told DH about it and last night we had dinner with J and her Dh and had a good laugh about it. Predictably N has not told me that she thinks I have the ‘hots’ for J and not her.

Any advice would be much appreciated.
Love,
ME