I just went through this with my DD a few months ago.

She had chronic Diarrhea, bloated tummy, farting alot (some of the signs of lactose intolarnce and Giardia). My Dr suggest I take her off milk (and replace with soymilk) and he also did a Poo test. It only took a couple of days for the results to come back and it was Giardia,gave her her ABs andI kept her on soymilk and gave her a probiotic for about 2-4weeks after the dose of ABs and she is now all better.