I'm wondering if I get a BBL if it's going to look unnatural?

Without seeing photos of you out of clothes it's not possible to give you any recommendations. Consult in person for an exam to determine if you are a candidate or not. Fat transfer is a great way to create a rounder buttock, but your final outcome depends on the quality of your skin and your anatomy.Best wishes,Dr.Bruno

Typically liposuction of the abdomen, sides, and back with fat transfer to the buttocks and hips produces a nice result. It's difficult to tell from the three photos (jeans/sweater) you provided.Kenneth Hughes, MD, Board Certified Plastic SurgeonLos Angeles, CA

Dear Ms. Lingglingg,Thank you for your questions and information.
Nothing substitutes for a personal evaluation and a face to face discussion of your options including a review of your medical history/medications  and a face to face discussion of your options.
Unfortunately with out nude  photos or a exam of your tissues I can can only comment based solely your photos. However, hopefully you will find them helpful.
The final shape and appearance depends not only fat injection but also shaping the buttock with precise injection of fat but also selective liposuction of the tissue around the buttock. 
The process for performing a BBL is the same regardless of the patients weight:1) harvest the fat 2) purify the fat3) inject the fat
The amount injected depends on how much fat there is available to harvest.One can try to gain weight prior to the procedure in hope that the areas you can the weight can be liposuctioned.
Caveat: not all fat gained can be liposuctioned.
Please make a appt. with experienced Plastic Surgeon who is Certified by the American Board of Plastic Surgery and ideally a member of the American Society for Aesthetic Plastic Surgery (denoting by membership as having met additional criteria and a focus on Cosmetic Plastic Surgery).
My best wishes,
R. A. Hardesty, MD, FACSDiplomate and Certified by the Am. Bd. of Plastic Surgerywwwimagineplasticsurgery.com4646 Brockton AveRiverside, Ca 92506(951) 686-7600