I have the exact opposite problem with my IBS. Taking strattera made it much worse. My doctor gave me levsin, however, and all is well.
It appears as though stimulant medications also affect seratonin levels as well as dopamine and norepinephrine. Seratonin is also responsible for involuntary muscle movement in the gastrointestinal system. In fact, prozac is often given in low doses to combat constipation. Likely the introduction of more seratonin into your system is causing the problem. Go to your doctor, there should be some medication they can give you.