Your symptoms sound most unpleasant! I'm sorry that you are dealing with this, and it definitely sounds like something you should explore further with your physician right away. Many medications can indeed cause constipation, including both of your medications. If you have been on the paroxetine for 2 years and this problem didn't really get most pronounced until after the bupropion started, then it is possible that the two agents together is part of what is causing this unpleasant side effect. It will be important to exclude an underlying issue with your GI tract--which is why you should see your doctor
about this right away--but hopefully this is a case of side effect management. If these two medications are helping your mental health significantly it would be nice if you can devise a bowel regimen that helps control your GI side effects but lets you continue on with the meds if they are working for you. However, it might be that switching the bupropion to another agent might help better control these side effects in a manageable way. This risk/benefit conversation will be an important one to have with your physician.
However, he or she will likely also want to check in with you about any other concerning symptoms and perform a physical exam to make sure that there isn't something else going on. Hopefully you'll be back to your normal self soon!