Hi Adrienne,
The answers are yes, yes and yes!!!
In my own personal experience I can tell you that there are some times when I have been able to pinpoint just what it was that sent me back 3 steps in my vestibular progress, and other times when I am completely clueless. I have noticed that I regress quite a bit when I have a cold... also, when it's "that" time of the month.... Other times, it's been the over-stimulation of a rich field of vision (the grocery store was horrible for me back in the beginning - in fact, I couldn't do it for several months), too much of a particular motion or head movement, etc. on and on the list goes.....
For some interesting info on compensation/decompensation process, take a look at the "sticky" information archive at the top of this page. Subs has posted quite a few things on how this whole process and what goes on as the brain has to learn how to compensate for the vestibular damage, too. It could be that this last session on Monday had you dealing with new VRT - and that's a new challenge to the brain, hence, the stronger decomp.
I have analyzed my many, many decomp. episodes - some I can figure out, often, though, I can't. I just remember reading something on this board repeatedly, though, and that's that vestibular healing is not a linear sort of healing. In other words, if I have a good day today, I expect to have another one tomorrow, and continue to improve as the days go by (kind of like you would if you had strep throat and then began to take the anitbiotics and started to get better - you'd just keep on getting better till the sore throat was all gone). But, unfortunately, this condition doesn't heal in that manner!
Please know that you are not alone in your misery, and that probably just about all of us who are on this board know exactly what you are feeling and how discouraging it can be. (By the way, you mentioned your gray, rainy weather, wondering if it might be contributory to your decomp - well, I'm in LA, where it's cloudless, sunny, dry and HOT and I'm still experiencing a couple of decompensating days

) WHO KNOWS!!!!
Take care, keep up with the VRT, and hang in there!!!! You're on your way!!!
Robin