I watched it at 1680x1050, you can see that this mark appears over her left breast ONLY.
...no mention this is shown too far from where her nipples are supposed to be and it hasn't any volume (is known she has big nipples).
That's probably oil or a similar substance.
Try to expand these images with Paint or something similar:
If her nipple is there, it is probably on image 4, but below and more at the right, quite near her arm (as usual).
I don't know if I'm able to post that video here (even being a public one), but I could share it here, so you could see what happens.