I switched from effexor to wellbutrin. I had to be on both medications at the same time for a long time because of the effexor withdrawal... it was so hard to come off the effexor, I was at the lowest dose for a very long time, 25mg, because I couldn't get off of it the withdrawal symptoms were too bad for me. Then I finally went to a compounding pharmacy to get an even smaller dose of effexor (I just wrote a post about it called "Effexor Withdrawal Suggestion') and that helped me get off of the effexor, it still took a month though. Now I am just on the wellbutrin and I am fine, no side-effects at all. Also, the effexor withdrawal symptoms did have nausea, headache, vertigo... as well as other symptoms too for me, so it sounds like it could be effexor withdrawal for you too maybe. Hope the withdrawal symptoms get better soon! I really like wellbutrin, I hope it works well for you too! :)
These meds are both anti depressants but they are different. In my opinion Effexor is more calming and helps both depression and anxiety and Wellbutrin is better if you just have depression.
You could be going through withdrawal or it could be the introduction of Wellbutrin.
Really though, I think it's withdrawal from the Effexor. Take care. Remar