It is 1000000% normal and generally unavoidable. Your hair naturally goes through growth and shed cycles and any major trauma to your body (surgery, stress, pregnancy, etc.) can impact that cycle. You just had major surgery and now your body is losing a significant amount of weight in a very short period of time, which triggers that shed cycle to accelerate. For most folks it lasts a couple months, slows down, and then you start to see regrowth.
Take your bariatric vitamins, hit your protein goals, and stay hydrated. Those are your best tools for helping minimize it and ensure regrowth comes in strong. Some people also supplement with collagen and biotin, which certanily can help, but they won't stop the shed.
Hang in there, its totally normal and doesn't last forever!