Any hormonal things going on? Meds that have changed; maybe the intense-to-nothing was a stressor in and of itself? And how has your sleep been? That alone can mess things up, even other than Fibro.

It is unfortunate that it's during your time off! But, like you say, maybe it's because you've given yourself permission to relax and pay attention to yourself, kind of like how after an accident/emergency is safely over, is when we come apart.