Believe it or not, "unstable moods due to a change in medication," as you put it, WOULD fly as an excuse. You could even get a note from your doctor to prove it if your professor doesn't trust you. Seriously, I just missed a MAKE-UP exam for instability, and my teacher has been fully understanding and accommodating. Talk to your teacher, and if he/she says no, go over their head, because it's a totally valid excuse.

Good luck with your transition, and I hope you start feeling back up to speed soon =D

~TheDeliciousDish