What is this? Large lump on butt after BBL? (Photo)

Hi, It is referred to as a "blow out" deformity. It is rare, but is caused by the increased pressure under the skin when fat is injected.

The fat necrosis may need to be removed if it persists over a period of months or is extremely painful. Sometimes the dead fat can create dents, and sometimes it can create a large unsightly lump.
Dead fat, unlike living fat, cannot be removed by liposuction, or removal of fat through a cannula by pressure extraction. The dead fat hardened by calcium must be removed in total through an incision in the skin, much like a cyst or a tumor is removed.

Dear Emilyg1234,
I understand your concern. However, without a proper assessment, it would be difficult to determine what went wrong. It is best that you visit your plastic surgeon for further assessment or ask for a second opinion. Only after a thorough examination, you can get proper recommendations and advice.
Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery

Hello @Emilyg1234, thank you for your question. Any kind of abnormality about the process must be analyzed, because there is no way to have a treatment if you are not clear about what is going on. The first step may be to do an ultrasound of the area. The best course of action is to consult one or several board certified plastic surgeon for more information and options tailored specifically to your needs. Best wishes!Alan Gonzalez MD, FACS. American Society of Plastic Surgery Member (ASPS), International Federation for Adipose Therapeutics and Science” Member (IFATS), Colombian Society of Plastic Surgery member (SCCP)

Thank you, Emilyg1234 for your question and photographs.
Photographic evaluation is incomplete without personal examination to give the most appropriate answer
However from the photographs, it seems that,
Sorry to see your condition. Kindly do sonography so that the cause can be determined and whether it is melted fat accumulated in that area or a cyst that needs to be removed.

Wish you an alluring look.

No one is going to be able to tell you some pictures alone with that mass represents.
I recommend you follow up with your provider to see the video she has to say.
To make an assessment and the outcome of a procedure we generally always need to see proper before and after pictures. If you don’t have before and after pictures then ask your surgeon to forward the before and after pictures they took.
You could get an ultrasound to confirm if it’s fat, necrotic fat or liquid.
Your provider should be helping you.
Best,
Mats Hagstrom MD