Butt gone flat after BBL. Can it be fixed? (Photo)

Did you lose a great deal of weight with your influenza? It seems unlikely that you would lose all the fat that was transferred.  Your initial results may have been fluid and swelling that has just subsided with your recovery. If you are not happy with the volume, the solution is to redo the procedure. 

It would be very unusual to "lose all your fat" even if you had to sleep on your butt immediately after your BBL surgery.  It is possible that your results would have been sub-optimal regardless. Another BBL is always an option assuming you have enough fat to donate. Make sure to do your research and find a Board Certified
Plastic Surgeon who is an expert at BBLs. He/she should do at least 6 per week
and be able to show you over 100 of his/her own before and after pictures.