Feeling guilty : /

So I got a call this morning (9:00 am) from my boss asking if I could come in today at work because a co-worker of mine is sick (it's my day off), I told her that it was really bad timing because I had a lot to do today and then she half laughed and said "well you'll get it done by the time you have to come to work" (which is at 3:15 pm.)

I swear to god if I didn't already have so much to do with my older sister coming to town on Friday and I having to clean my place and stuff and run a couple of errands in town I would have said yes in a heartbeat, the only way that I got myself out of it was saying that I had an appointment at the hospital and even then she was very persistent about her working until I was done with my appointment and I guess it's a part of why I feel so guilty, but i'm also wondering why she can't call ANYONE else.

I mean there are like 10 other people she could call if they don't have to be in at 3:15 - 4:00 pm and i'm really wondering why she was so determined about me?

Should I feel guilty, am I a bad person?