Is it a sin to want a day off?

Sorry. I just have to vent.....I work 2 jobs. I worked my primary job the past two days, horribly busy, 14 hour shifts. I'm off today. Work the rest of the week. Yesterday AM I was asked to come in to do a C/S. at 6:00 AM today. At the time I felt like I was getting sick plus I had alot of things to do today, my only day off. So, I said no. Then when I got home last night at 9:00, I had a message asking me to work this evening because they already had a call off. I said no, I had plans for today plus I wasnt feeling well. Now I have a message on my machine stating "We're really crazy here! Could you help us out?" Why do I feel so guilty? There are 35 other nurses they can call. Obviously they don't feel guilty saying no. I feel like I don't have a good enough reason other than not feeling well, and having a million things to do. Am I wrong? Does anyone else feel the way I do?