Am I Being Taken Advantage Of?

I have a co-worker that I'm cool with, and we get along great, but she keeps asking me to work her shifts for her. The thing is, she doesn't even really need a day off, she just doesn't want to work. First, it was one day, and I was fine with that, no problem. But then, it was another day, and yesterday, a half day. Then just now she asked me if I could work Labor Day for her, tempting me with the whole time and a half. Honestly I don't care about that, because it's not much of a difference on the paycheck, in my opinion. To me it's not about the money, it's the principle. This is the fourth shift she's asked me to take for her this week! I will take shifts for my co-workers, but when it becomes continuous, I begin to question if they really need a day off or if my kindness is being taken for weakness. Please tell me what I should do in this situation. Any advice would be appreciated, thanks!