Hot red golf ball lump on left buttock (Photo)

You should see your surgeon immediately to assess whether this could be necrosis or an infection. It is best to be treated sooner than later. Regards,  

Since about half of transferred fat with BBL dies (in every case), you undoubtedly have fat necrosis in some areas that were grafted. One of these areas sounds as if it is infected as well.Antibiotics are likely necessary, but will not "cure" an abscess--surgical drainage will be necessary for that. This should be tended to urgently, as in ASAP! Please see your surgeon immediately. Best wishes! Dr. Tholen

I'm sorry to hear that you are going through this.  There is no way to know just from your picture and description whether this is a fat necrosis or an infection or what.  Since you have already contacted your personal plastic surgeon and since there is a darker area in the skin, a reasonable assumption would be that you have an abscess.  Very often, these present without any fever.  If your surgeon does not respond within a couple hours, I highly recommend you go to an emergency room.  This may possibly not need emergent treatment, but I wouldn't take the chance.