Yes, this happens to me all the time, you feel great while doing whatever and then it hits you and you are wondering what the heck is going on!
I just drag myself to a chair and rest, and drink water, drink lots of water I really think that helps me, because the old stand by caffeine doesn't work any more.
If it is hot, get in front of a fan and rest, that is all that you can do. I know how frustrating it can get when you are used to being busy, and doing everything yourself, but learn to say no or I can't do that right now and just realize what you can do, and pick your battles. It is hard but it will make those cotton headed times not occur so much, and it gets easier
Don't worry you are not alone!