Maybe it just hurts a little, because of that skin tag she has on her butt hole. I've had a couple of those things ripped off of FAR LESS sensitive parts of my body, and it hurt like hell, and bled worse. I can't imagine a constant friction on that is gonna feel good, especially from a porn sized cock, and I can't blame her for not wanting to go to a doctor and getting it removed...if you even could.
And of course I should point out, I don't bring this up to try and make Brandi feel bad, or self conscious, but it is a logical explanation, and it is clearly visible.