Sounds like she's TRYING to use her current babysitter, and that one is having none of it - and is leaving, period, and will probably call the cops if mom (or someone) isn't home to relieve her.
As a parent, you just can't do that... shove your kids off on people and come back 5 hours late. Sounds like lady needs a serious wakeup call.
Personally, I wouldn't want to insert myself into that drama NOR set myself up as the 'booty call babysitter' -> call last minute, any time of the night and I'll come running. If mom has another sitter, the sitter could well have already CALLED the cops on absent mom.
Now... I could be totally wrong, and mom's totally on the ball, and there, and that was a one time thing... but since you're asking if you're being used; you probably are.