Welcome to HealingWell and the bipolar board. I can completely relate to what you are describing. The medications do stop working periodically, and then you need new doses or new medicines. It sounds like the Wellbutrin is managing your depression pretty well, but flattening you, or your depression is creeping back in. Another med might not do that. You may not want to change right now with the baby on the way (Congrats BTW) but afterwards be agressive about monitoring your moods. You don't have to live flat, but you don't want to get depressed either.
Don't discount the power of the hormones as well. They can influence your combo in untold ways. You're vulnerable right now.
Definitely get with a psych and talk this out. They may have some solutions for you.
Glad you're here,
Co-Moderator, Bipolar Forum
"Bipolar disorder can be a great teacher. It's a challenge, but it can set you up to be able to do almost anything else in your life." - CARRIE FISHER