Lactose Intorlerant?

It may look like my 5 month old baby girl is lactose intolerant, we get have weighed every week at our health nurse, but never puts on that my weight, she can put on 150grams in 2 weeks then lose it the next too weeks, i am feeding but had been told to start comp feeding her with formula after feeds, but we have tried two kinds of formula and she just vomits it all up 2 hours after her feed of it and the the next hour she is vomiting flem up, I saw my health nurse today and she was the one that said she could be, but we are trying one more formula to see if she does the same thing, and if she does then i hav to take her to the doc to get tested. If she is lactose intolerant, could this be why her weight fluctuates? Her nurse also said she does need to put on more weight as she is nearly six months and doesn't weigh 6 kgs. Her dad was lactose intolerant as a baby too, how common is this?? What happens if she is??