Having good days, is a good sign. Having no good times is a bad sign.

It means your trigger is not constant.. If you have spinal compressions, then over activity may trigger you.

Something in your environment. That is why keeping a log is a good idea...it might pinpoint something you hadn't thought of.

Big stress actually depresses the immune system. People under a sudden stressor can get flu, infections, pneumonia, etc when if the stress hadn't happened, they would not. There is a link to infectious illness and the hypothalamus...
more here:
http://en.wikipedia.org/wiki/Psychoneuroimmunology

We just had a medical emergency with our young 2 yr old cat this summer, and the Vet on vacation explained all this and how CATS have acute illness following stressors! So it is not just us! She had a run in with a mink we think, had an invisible bite on her back neck area (mink have short teeth), and went into a lethargic coma state over the first weekend we were up there. She almost died! Hence the Vet lecture! After almost a week the bite festered up and then the treatments were more clear and she did recover. But it was very dicey for 3 days or so. I had to give fluids by syringe orally every 10minutes! She had a really high fever too. So it might be a mammal thing...stress, depressed immune response, illness and perhaps worse for some.