I can relate to how you're feeling - it's so frustrating when you don't understand why. Have you been seeing your dr regularly and telling him about this??
I had to try 4 different ADs before I got onto the right one, and even then after 12 months or so the mood swings started up, tho not the deep depression, so I was put onto another medication that helped with the mood swings. "Touch wood" - I haven't felt this good for a long long time and no sign of falling back.
Please speak to your dr about this and get him/her to consider adjusting your meds, or at least reviewing. Do you have CBT? This often helps too, giving you strategies on how to deal with these mood swings when they hit.