I would say it is one of the following:
Bad latch/hickey - DD occasionally gets a mouthful of areola instead of the nipple and she can raise a hickey in less then two seconds, and it isn't painful or sensitive after the fact
A cluster of spider veins - this would be caused by the stress on the tissues due to the fluctuation of engorgement and "empty breasts"
Bruise cause by engorgement - this is very unlikely, but possible; if you get engorged fast enough, the internal tissue could be stressed beyond the norm
If it doesn't change (except for fading) or reoccur, I wouldn't worry. Our breasts take quite a beating during the first few months of b-feeding.
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I'm a head-covering witchy mama to DS ('06) and DD ('10) with DH, Stormie, a Heathen breadwinning daddy.