No one in my immediate or extended family is lactose intolerant except for me. how common is this? had loads of tests ruling out celiac, ibd, etc.

Lactose intolerance: It does not need to be hereditary, it is pretty common.

VERY Common: It is common to have lactose intolerance, even if it does not run in your family. Luckily it is easy to avoid those foods.

Very common: Most of the world has lactose intolerance actually. While there is a gene that controls the enzyme responsible for breaking down lactose, there is a lot of variability in the way people react to lactose, so it is not surprising that other members of your family have not complained about GI symptoms after eating dairy.