I would hazard a guess that as well as PMT, your daughter may also be suffering from depression.
It is hard when you feel as if you have been cornered into a life you never planned for yourself. As much as she loves her son, she may feel that her whole life revolves around him and there is little left for her. I know from experience.
It isn't your fault that she is like she is. Blaming someone else is so easy to do when you can't see any way out or any way that your life can change.
The hours that you are working I would guess means that she is on her own for most of that time - you don't say that she has a job herself, or how old your GS is but when you return from overseas, it might be an idea to sit down with her and work out a sort of shift system around your GS so possibly your daughter can work while you are at home with him? It might boost her self esteem and help things all round.
Oh, and make sure that if she does have a lodger while you are away that they sign an agreement or you may well find you are stuck with an unwelcome "guest" that you can't get rid of !
Another welcoming into Soop's kitchen for those who care to share...