What Causes Vomiting And Loose Motions In A Child?

I have 20 months daughter who vomits in night once or twice a month while she is asleep then after few bouts of vomitting she sleeps but next day she starts with loose stools which are curdy, what could be reason for this? Doctors have prescribed Zifi 50 and Bifilac. Once it is started she feels better in one or two days. but i am worried about why she vomits at night, Doctor has also saying she might be having GER problem but still m confused is it becoz of GER or infections she is falling sick...pls reply my query whether zifi is required or only pro biotic will suffice...thank u Doctor