Acid Reflux/Heartburn after Using the Prograsm

I'm a total noob to the Aneros and have started with the Progasm. Each time I've used it, after half an hour or so I start to get acid reflux and abdominal discomfort. I've tried searching other forums to see if anyone else has experienced this sort of thing but haven't found anything. Was wondering if anyone here has. If so what advice do you have? As far as I know, I'm using the Aneros correctly, lying on my side with my knees up. I can't for the life of me figure out why this is happening. I've had four sessions and each time it has happened without fail. Any feedback is welcome.