PMT is such a horrible one to live with whether you are the one suffering or the one living with the sufferer. If she is anything like me, she will feel fully justified when in the PMT zone even if she is being totally unreasonable and then, when she's out the other side, she beats herself up and feels unloveable because she knows how she's been. The less she loves herself, the more she'll look to see if she really is loved particularly when she's back in the PMT phase and if you look hard enough, you can always find them. I became quite paranoid during those times. Meanwhile the people living with you feel they are living on a knife edge and PMT lasts longer than people realise so you never really get back to normal before its starts rearing its ugly head again. Antidepressants helped me live with it for years until I had a radical hysterectomy.
From what you say, it seems that you are either "involved" with your DGS's care to give your daughter a break or you aren't so she can't cope. Is there a happier balance you can make? Perhaps get it so she asks for when she needs some time off but you take a back seat. When she is in a good phase, ask her how you could make her feel more in control. You might be surprised at the answers you get. Take some time to think about what she is saying with a "I'll have to take some time to process that," if you think she is being unreasonable. You can always return to her with a well thought out compromise. You might have to do this several times to affect any change but the moment you signal that you need time to think and reflect on what she is saying, you are acknowledging that you are listening. Too often we get caught up in arguments because we don't give ourselves time to process what the other person is saying and respond instinctively when you may actually have to reframe your instincts.