wellbutrin pill casing & nausea

I started on Wellbutrin xl about 10 years ago and took it for 5 years and suffered from nausea the whole time. Then I started on Watson's generic buproprion xl which has a different pill casing. I didn't realize it, but the nausea apparently subsided with this change. I recently switched to another generic (Anchen) with the same type of pill casing as the Wellbutrin and the nausea came back. Has anyone else experienced this? Is there a difference in the time release of the two casings? (I have not noticed any mood changes with these med switches) thank you