I'd suspect it may have to do with your adrenals being slow to wake up after relying on the prednisone for several years. I've read many reports (on the HealthUnlocked PMR site out of the UK) about the symptoms people have when they get down to the dosage you are on...depression, exhaustion, mood swings. If that's what it is, you just have to tough it out until your adrenals are fully functioning. The good news is that it's temporary (but it may last longer than you'd like) and that you're only on 5 mgs.