Actually, I think it's a body double's breast we see.
It's quite common in movies to use a body double to show a person's naked butt or breast, or vagina, and not their face at the same time.
Now, if the shot had been further back and had both her face and naked breast in the same frame, then I'd believe it was really her breast.
The same thing was done with Julia Roberts in "Pretty Woman". The scene in the beginning, with Julia in bed waking up as the song "Real Wild One" was playing, and that 'perfectly round' butt shot, used a body double. It wasn't Julia's butt that we saw. A body double was used in the shot, and we didn't see Julia's face in the frame.
The trailer has all that is good in the movie in it, and is both funnier and better-paced. If you keep hold of your money I am sure there will be other and better movies which feature, ahem, "more" of Ms Bloodguard