Jacuzzi Gross-Out (Possible NSFW and NFBSK)

[Rumiko]

It is titled as "Jacuzzi Fart" however, I really don't think that's what's going on.

http://www.youtube.com/watch?v=iiNNUggNnHQ&NR

I don't see what initially shocks her at first; I don't see the stain come out as before she reacts. It seems she is holding her stomach when she gets up (maybe she felt it, maybe she's in pain?), which led me to believe that maybe she is on her period. Can someone bleed that much in a few seconds?

Any ideas?