I am 8 weeks post-op BBL/Lipo, what are the symptoms of fat necrosis?

Symptoms of fat necrosis vary. Anything from decrease of volume, seromas, to an infection... usually fat necrosis can be diagnosed with a clinical exam or with a test of ultrasound or MRI scan. 

Fat necrosis, whether in the buttock or elsewhere, usually involves the feeling of a hardened discrete mass that is the calcified dying or dead fat.  Treatment may include massage, tincture of time, as most of these go away with time.  Occasionally, these areas may have to be excised.  There are probably multiple reasons for this to occur, some of which are not entirely preventable.