For many people chronic back pain varies significantly over time. You'll have good days, good weeks, and good months, just like you'll have bad days, weeks, and months. Usually the bad periods are caused by some sort of unfortunate event (for me, air-travel is the most dependable cause of a flareup). When you have a flareup there is a perverse desire to start doing exercises and stretches right away - the very exercises and stretches you should have been doing before you injured yourself to reduce the chance of a flareup.
I've found it's very important to resist this urge. Once you have an inflamed back the surest way to make it worse is to stretch or do other unusual exercises. For instance, the two times I've tried yoga while my back was hurting, I've ended up in significantly worse pain afterwards. The same is true of times when I've tried to get aggressive with my physical therapy exercises. It appears that injuring yourself is much easier once there is some sort of acute inflammation, and many things that might normally be helpful are actually quite harmful.
This may be the reason why bed rest used to be prescribed by physical therapists. That is not the solution either. Moderate exercise actually speeds up recovery. The trick seems to be that doing the sort of movements you make every day is OK, because you know how to do them in a non-stressful way. This may be because you know from years of experience exactly what those movements should feel like, and therefor can detect almost instantly when a particular mode of movement is too much, and needs to be adjusted. For instance, going on short walks actually makes my back feel better, at least when I make sure to move very slowly, taking extreme care that each step is pain free (or at least as pain-free as possible). Because I'm far from a yoga expert, I cannot make the same sorts of judgments about the safety of the yoga poses.
So the question is, when to start doing those exercises that you so desperately want to start again, in order to help your back? The key point, I think, is that those exercises are not meant to help you recover from an acute problem, but rather to strengthen you so as to prevent future problems. So there's no hurry to start them when you are still in the acute stage of a flareup. I would advise waiting until your pain level is no longer getter better on its own, which suggests that the acute phase of the injury is over. Since pain tends to vary a lot, day to day, a good way to asses this is to ask yourself if you are significantly better (or worse) today than you were a week ago. If you can go a week without any change in pain level, then it is time to start, very cautiously, with your exercises, stretches, etc. Otherwise, what will undoubtedly happen is that you'll start to feel better, and after a day or two, launch yourself into some sort of activity which seems like it should be safe, only to find yourself in much pain later that day.