Girlfriend uncomfortable during ?

when i have with my girlfriend, she doesnt really enjoy it because she says that it feels like there is a 'weird pressure' or 'air in her ' and she tenses up in an attempt to stop her from 'fanny farting' and turning me off. when she does that she says it doesnt feel very good. i want her to enjoy as much as i do. does any1 know what this pressure is, how to stop it, if it will actually result in a fanny fart, or how to convince her to relax?