Why does my girlfriend get mad when i...?

fart or have gas. she acts as if she doesnt ever do it. like if were on the fone and i do it and its loud shell be like omg really??? dats gross. as if she never does it. or if i burp shell be like eeew whyd you do that. why do you do it when your not around me??? then like sometimes when were riding on the bus together or something and i fart she instantly gets pissed but i find it hilarious. shell be like thats disgusting and ill just be like no dat sh*t is funny and i have a good laugh about it... like you should. it annoys me how she acts like she doesnt do it tho. how can i make her comfortable enuf to do it around me