I was wondering if this is the beginning stages of tissue necrosis? (photo)

You hand is covering the areola, and the area you are displaying looks to me like it will survive. Nitropaste works immediately upon application, it just needs to be applied min of 4x/day. Wait and watch for now, most likely this will resolve without any further intervention, but keep in close contact with your PS.
Best.

It appears that you are suffering some degree of tissue necrosis but it is not uncommon for smaller areas to require delayed healing after a breast lift or breast reduction. The area affected in your case seems unusually large but a lot of healing will occur with proper wound care management. I personally do not use Nitro paste but would have you applying either antibiotic ointment or mupirocin ointment to protect the areas and encourage healing.  Do expect the healing process to take several weeks but be patient with it eventually you may or may not want to undergo scar revision a much smaller procedure. Good luck and best wishes,Jon A Perlman M.D., FACS
Certified, American Board of Plastic Surgery
ABC-TV/Extreme Makeover Plastic Surgeon
Beverly Hills, California