Is this ischemia? I'm 4 days post op a breast reduction. (photos)

My surgeon did a great job. My nipple is dark around edges and nipple but will refill after I press on it. I have an app tomorrow. He gave nitrobid 2x daily after I text him which I started tonight. Also, since applying it I feel like there's a sort of throbbing or warm sensation every so often, does this mean it's working and will get better not worse? At 4 days post op is it being caught early enough to repair as opposed to prevent further damage ?