So: what the heck is going on? It's been a really tough first quarter, we'll just about hit plan and at long last I got a week-end where I could spend some 'down' time. And what do I do? Feel ill, really ill. And it happens too often to me: when I stop, I fall ill. How does that work?
You have a body which is pretty smart and which is trying to protect you from over-work. When you work too hard the body assumes there's a reason (fight or flight etc) and will do it's best to support you (only up to a certain point of course). But once you stop it will try and use that time for recovery. At it's simplest that might display itself as exhaustion or possibly as illness as the body goes through a healing crisis.
Bottom line: your body is telling you to slow down. Before it is too late.