You can get gastric fluid in your sinuses, but I don't think it's that common there is no flap to my knowledge that would do that. If you get heartburn at night, the fluid gets up in your throat and if you are lying flat it can travel from your throat freely up in to your lower sinuses. Is it at the back of your throat or does it feel like it's farther down in your throat? I would start out by trying one thing.. Try sleeping on an incline. That doesn't mean you necessarily have to lift the bed up some, but maybe get some kind of wedge pillow or stack multiple pillows so you sleep with your head slightly elevated (more than a regular pillow). This will prevent any acid from ever getting into your sinuses (if that's actually happening.. But this will rule it out). If you sleep like this for awhile and it doesn't help, then it's not heartburn. However, you can get heartburn at night and not even know it so even if you don't feel it during the day you can still have it. Heartburn can also give you a full feeling in your throat. I've been suffering with gerd for years now.