Doggy Style

So my boyfriend loves having sex doggy style with my hands tied behind my back, and I love it too but there’s a problem. We can only do it for a short period of time before it starts to hurt my stomach and my vag fills with air and I start to feel sick and then my stomach fills with air and when ever I tighten my abs it makes this weird grinding sound from the air moving around (not like queefing nothing comes out until I sit up). Any idea what could be wrong? Are we not doing it right?