Personally, I don't think your GS occasionally calling you 'mama' is the problem here. If your daughter is having to go away on work related trips for up to 4 days at a time, then she is very lucky to have you to help out with childcare. Although, the way that you've worded your post, it sounds to me as though these trips are not work related, please correct me if I'm wrong ...?
If she's going away with friends then I'm afraid that you are simply enabling her to do so, and the fact that you're also providing childcare every weekend, it sounds to me like she's taking advantage of you, particularly when you say, 'It's a lot on me to watch him for such a long length of time while she goes places without him'. It absolutely is a lot to be expecting of a grandmother!
Do you not have a life of your own that you would like to spend time at weekends with your partner (if you have one), or with friends/other family? Even if you don't, I think your daughter is asking way too much of you to expect you to take care of your GS every weekend. She seems to be shirking her responsibilities as a mother.
Does your daughter have a partner, or is she single and wanting to go out every weekend with her friends? If she does have a partner, then I think they both need a darn good talking to!
Whatever her situation, she needs to start taking her responsibilities as a parent more seriously, but that's not going to happen if you continue to enable her to take advantage of you, and to go galavanting whenever, and as often as she pleases! ...