My gf has a really lovely, sexy bum so during foreplay I love to have her lying on her stomach naked while I lovingly and passionately massage her buttocks - however it nearly always causes her to fart. She usually giggles and I laugh it off too, but sometimes it gets a little bit annoying and offputting, and kind of ruins the mood slightly (and sometimes it's kinda stinky). Why does she fart when I rub her butt? And is there a way we could avoid this happening? And is this normal?