When this happens and suddenly in the string of good days comes one or several bad days, I just try to work and be at home and generally avoid anything that might upset me.
It's usually good to have a few books and nice simple chick flicks at the ready somewhere. Also, going for a run helps. When you actually make yourself get out of the building and make the thighs burn.
I don't try to analyze the slide downs too much and just sort of blanket myself when they come until they pass. For me, these lows come mostly in the evenings and the ADs have improved my sleeping by leagues so that I can sort of protect myself by going to sleep early as well. I hope these tips will be of any help to you.
And if you have a pet, cuddle cuddle cuddle.
Good luck on fighting it.