Rant, rant, rant

OK so here's the problem:
I babysit my niece and nephew from Saturday to Wednesday fro 2 to 12. The other day my sister threw her knee out so I was like you know I'll stay the night and watch the kids so she doesn't have to get up when my niece wakes up in the middle of the night. Since I was doing that i couldn't go to sleep till about four in the morning because I sleep like a rock so if she cried I wouldn't hear her all the way in the living room. Fine. So that night I got a total of two hours of sleep. The next day my sister goes to the hospital for her knee and she gets a brace so I spent the night again that night I got a total of three hours of sleep. The next day my sister has a knee doctor apt so I stayed and babysat. That night I got about six hours of sleep, which was more then the other two days but I was still running on 12 hours of sleep for three days.
OK now to the problem. I basically babysat for 72 hours nonstop for free (I get paid $80 every two weeks) I dint ask for any extra money or anything like that. So I come home yesterday and get the first full nights sleep in three days and then woke up and had a lovely thanksgiving with my family. After dinner and we came home I sat on the floor and played some Call Of Duty for a little while. In he middle of the game my sister calls me and asks I can babysit today. My answer was no. Thursday and Fridays are my only two days when I can get stuff done and most the time I go over there and babysit for free on my off days. So am I wrong for saying no? I didn't have kids and they aren't my responsibility. I'm a 17 year old girl who can't even spend the night over her best friends house because she has to work on the weekend. I have pretty much given up my whole entire social life to babysit and she gets an attitude when I don't want to babysit for one day? I have the right to sit at home and stare at a white wall all day on my off days don't I? I have the right to say no, right? I'm not a horrible person because I charge my sister to babysit her kids am I? If this is my supposed "job" I have the right to get pair, right?
Maybe I'm just ranting but I feel really really used right now. I can't go out and have fun because I have to watch two kids that's rent even mine 24-7. I have a right to actually be off on my off days don't I? Then why do I feel so bad for saying no to babysitting? Like I said before I just feel used.