My dosage was lowered exactly the same as your's, that was on the back of my annual blood test results. I also quite often fall asleep in the afternoon though I've never associated it with the thyroxine.
Was your dosage reduced after a blood test ? If it was, I'd just go with it for a few weeks. Far as I know, any change to the dosage takes about 6 weeks to make any difference anyway. Could be the weather, or getting that bit older that's causing fatigue ...
If you haven't had a blood test recently, get one done (tho' think you must've for the dosage to be changed ;) ).