Hi Jesselou
This is caused by the size of your uterus.
When you are laying down your userus is pushing up against your stomach and causing the stomach acid to flow back up to your throat. It is also pushing up against your lungs and giving you the breathlessness feeling. Try propping yourself up a little more as MantaRay suggested. You may also find a small non-fat snack before going to bed helps with the acid.