4 weeks after BBL and lipo and I already lost 5 pounds, why?

So I would venture that the weight loss you are experiencing is mostly loss of fluid (likely from fluid that was used during the procedure). If that is the case, your weight should stabilize soon. Fortunately (or unfortunately depending on your perspective) the swelling that you likely have will also go down over the next several months. Results typically stabilize in about 6 months in my experience. As always it's best to speak with your surgeon.
I hope this helps!
Good luck! 

At 4 weeks out, your swelling is subsiding and with that you might drop a few pounds.. ...............