Why is this happening????????10:points best answer?!

If you're a little on the heavy side then that could be your answer.
You could possibly have too much weight or too much to support when your on your back thus your lungs wouldn't have as much space and ability to inhale and exhale oxygen.
When you're on your side there isn't much to hold your lungs back from taking in as much oxygen as it needs.
Being on your side is probably causing your body to make an opening somewhere or a passage to let the air flow in faster, versus when you are on your back.
If you're a smoker that can be it too,
tons of people develop sicknesses or symptoms due to smoking and it does cause you to not be able to breathe properly, it slows down your breathing or makes it extremely harder.
Your basically weaker on your back and have a hard time lifting up all your body parts when you inhale oxygen cause remember when you breathe in your stomach, rib cage and all that go up, right?
So maybe it's just that, you probably need a machine to help you pump oxygen or do aerobics (helps your breathing).