I get air in my vagina???

Every time I try sth with my boyfriend like doggy style or generally him being in my back I get soooo much air in my vagina that it starts hurting after a few seconds and I have to stop him, get up and press my lower stomach so I can push all the air out. It never happens if I'm on top or misionarry, or any other thing that doesn't imply him being in my back. Does this happen to anyone as well? Is there anything to do about it? Could it be because of the condom?