Just that really. For the first day or two (and often the day before) I really struggle to concentrate. I get this 'brain fog', whereby everything takes 10 times longer, I'm really scatterbrained and I just don't trust my judgement. This makes it somewhat problematic at work. I'm virtually useless. It's really quite debilitating!
I don't remember being like this before I had kids. My periods are so much worse now!