Is "a sense of impending doom" a symptom that something could be wrong?  am taking bupropian and tysabri (natalizumab). Should I see a dr?

See a doc. A sense of impending doom is one of the symptoms of a panic attack, though you do not mention other symptoms. First see your medcal doc and if all is well, see a psychiatrist.
Something is wrong. A sense of doom is indefinite. Is there associated shortness of breath, elevated heart rate, headache, or faintness? Wellbutrin (bupropion) can occasionally cause anxiety - even in those not predisposed. There does not appear to be a problem with interactions between the two. You might consult the md who prescribed the wellbutrin (bupropion) - maybe try something different, a lower dose, or XL form (? The buffer cause).