Pregnancy definitely affects the immune system, but it is generally a suppression of immune responses. This makes sense, so that the body does not perceive the fetus as being foreign. Nonetheless, in some women this could cause a re-set of the immune system that promotes auto immune disorders, or at least immune dysregulation. I don’t know the data with regards to prevalence of these disorders among women who have had pregnancy or not, however. Nonetheless, people such as yourself who are afflicted with some form of immune dysregulation must also suffer the frequent indifference of the medical profession to dig deeper into your “why”, because the expert mindset prefers to operate within its comfort zone, with what it knows, rather than what it doesn’t know.