Let's see, first I'd ask if you're taking them with food? And, believe it or not, it matters if you take them BEFORE or AFTER you eat that food. (Honest: for a while, I had to take one of my meds just before I ate breakfast -- sit down, take pill, pick up fork, etc -- and the other AFTER eating my egg, BEFORE eating my toast...)

Also, try taking the Wellbutrin in the AM and the SSRI in the PM. Especially with Lexapro, I'd think taking it a little before bedtime might be a good choice. (Made me sleepy, at least.)

The bottom line, though, is to talk to your doctor. SSRIs can cause GI troubles, but those effects tend to fade after the first couple of weeks. If you can get through those first couple of weeks, you should be OK.

Good luck.