This is reminiscent of traces of injections.
Sex, drugs and rock'n'roll?
It looks to me like the mark a drip leaves behind. Perhaps she was in hospital or suffering from dehydration a few days before shooting that scene?
she was in the hospital. That happened a year ago.
Well, we now have four straight releases in which she evidences this weight gain.
http://ilovebuffy.com/photocontent/funonstairs/stairs10.jpg
Another proof (as if we didn't have plenty already) that she's been retired for almost a year