I am a nurse, and was totally unaware that both Giardia itself and medications for Giardia can CAUSE DAIRY INTOLERENCE/ALLERGIES! It seems that many folks recommend that you go off dairy products for at least one month during and after taking Giardia meds. I don't know when you finished the last course of meds, but you should try staying off dairy for 4-6 weeks, and then slowly re-introduce dairy products. With luck, the symptoms are "passing"
I do not know the accuracy of this, but I read in several sources that yogurt with active cultures does not cause the distress and symptoms that other dairy products cause during/after treatment. Worth a try!
BTW - We just got another positive culture back, and are starting on a second round of medications today!! Back to soy milk for us as well...!