baby's digestion

i have a 3.5 months old baby about 4 weeks ago his stool (kaka) began looking greenish, plus it is almost comletely watery. sometimes it is more like slime. i am nursing exclusively and feel well in general. the doc. said this can happen but that he might have parsires or some kind of salmonela. i am not able to take samoles since it is watery so i am left puzzled. do you think there is a need to worry? by the way, he is gaining weight and generally seems in a good mood (with the exceptions like any other baby)