Medical student's apparent celiac disease responded to giardiasis treatment

So this is actually shockingly similar to something that happened to me. I did a lot of back country travel with sketchy water and developed something close to dairy intolerant for more than 10 years to the point I wasn't sure if I had always had it. Eventually I got lucky on a test (for E. Coli not giardia) and the treatment was to go to nuclear on my intestines. That week wasn't fun. Now i can eat ice cream and dairy with out trouble. I'm not convinced that test was related to the 10 years of symptoms, but the hard reset on the gut definitely worked.

Interesting, I have some kind weird dairy intolerance that isn't just lactose intolerance (I have that too), but some kind of sensitivity. When I cut out all dairy my weight jumped 25% in 6 months, and I was no longer underweight or feeling chronically sick. I had cut out lactose heavy dairy years earlier (soft cheeses, milks), but still would have hard cheeses and lactose-free products.

I have no idea what is causing this and my doctors have just shrugged. I have no idea where to even start with this.