Why does most women hemoglobin level drops after having lipo and bbl? What can I do to prevent it from happening?

A low hemoglobin count can be due to blood loss, which is atypical after having lipo and BBL.  In an otherwise healthy person without anemia or other consuming disease, there is usually nothing that needs to be done.   Your PS is in the best position to explain why your levels may drop.

Dear Gorgeous4942,
Every surgery causes bleeding, which can cause a drop of Hemoglobin level. If you are considering a surgery, I would suggest you to consult a board-certified plastic surgeon. Only after a thorough examination you will get more information and recommendations.
Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery

After any liposuction, there is some blood loss (usually minimal).  Postoperative blood tests (CBC-Complete Blood Count) tests for anemia and several other factors.  Low blood counts found after surgery are the result of small blood loss combined with an increase in fluid throughout the body which can 'dilute' the blood and lead to a diagnosis of anemia (low blood count).  There is nothing to be afraid of unless your blood count is low going into the procedure, or if you had unusual bleeding during the procedure.  If you are asymptomatic (no shortness of breath, increased heart rate or low energy) then you should be fine with eating properly and if concerned, taking Iron and Vit C.Dr T