How can bacterial overgrowth , a common consequence of gastric surgeries , lead to changes in bile acids that upset bile function ?

Dysbiosis possible: It is equally possible that the antibiotics you took made your normal flora out of balance. When you have major surgery you are given broad spectrum antibiotics to prevent infection. But the antibiotics kill the good bacteria is well as the bad bacteria, then they get out of balance. Sometimes if you take probiotics, that can restore the balance and make things better.