I started taking wellbutrin (bupropion) sr yesterday and I am having constant heart palpitations starting in the evening. I have a mitral valve prolapse.?

MVP. It is not unusual to get palpitations when you have mitral valve prolapse. This make your heart more 'sensitive" to outside triggers and may make symptoms worse. Talk with your doctor to see if you can change from welbutrin to something else.
No way to tell. I will recommend that you stop taking wellbutrin (bupropion) sr and call your physician who prescribed it and inform him/her about the heart palpitations. Mitral valve prolapse is not the first in my list of differential diagnosis.