Well not haveing tried your reg in this situation, I would not know if it is a normal event, or if you have a defective valve, or an obstruction in the valve seating area.
NetDoc may be right with the pool trash though. It doesn't take much for the valve to leak when under pressure. A piece of lint or eyelash hair is all it takes. This leak would be more noticable when at deaper depths though.
I often hang inverted, or am on my back for short periods of time (5 - 10 min) when working. Every reg I have tried leaks in that position to some extent. When on my back, it flows right into my mouth. Lucky for me it's always fresh pottable water I'm working in. If I exhale slowly when on my back, the water comeing in does seem to increase, this is only because I am not exhaling with enough force to drive the water out with the air. Remember, when you are on your back, the weight of the water is resting on the valve, when it opens for you to exhale, the water is very eager to get in and does. When you are face down, the water will stay pooled on the valve area.
Have you reg checked out at your LDS. If it gets a clean bill of health, try a little experiment when lying on your back. Try exhaling slowly, and hard, and try to note the difference in the amount of water that leaks in. One other experiment you can try, is to lay face down, and inhale very very slowly after a hard exhale. After you inhale, role on you back, and see if any water entered during your inhale. If you feel that a sizeable amount of water did enter, then you most definately have a problem with the exhast valve and need ot get it serviced. Finally, please do not try this in open water without a buddy with you who knows exactly what you are doing. It would be dangerous alone, and would freek out your buddy when you started to "goof around" under water. Take your time, and soon you will know what to expect from you equipment.