When We're Happy

My mother has been here almost a week now and she had gotten to experience our son at his best.....and worst. See for the most part both babies are very happy. Our daughter is very calm and easy going when she is around people she is used to. Our son can go from happy to just completely unhappy and screaming. The problem is that I cannot figure out what causes him to be unhappy. Part of it is that he wants to be held more. On the surface this sounds fine, except I have 2 babies to care for and our son is a chunk!! I can usually distract him and move him to another toy and that helps him to "get happy" again. Hopefully this is just a phase that he is going through.