Yes I've noticed this. Some days I find mental arithmetic to be very difficult indeed. But not only that - it also corresponds with a reduced ability to plan tasks, difficulties in finding the correct words when writing or speaking, and social blundering in that conversations don't flow as well as normal.
I'm not really sure what it's all about but it seems to come and go. This is also on a background of a constant headache that I've had for almost twenty years; it increases in severity when I'm stressed, and I get stressed when I can't think properly (or perhaps, vice versa), so maybe that's related too.
I hope you can figure out your problem. I'll be reading other replies to this question with interest.