Actually, it's come up before that people have acid reflux and don't know it, because most people think acid reflux equals heartburn, when nausea is actually just as common a reaction. So that may very well be your problem, and may be helped out with some OTC acid reducers and sleeping a bit more upright in bed.
Sinus drainage down the back of the throat can make you feel sort of gaggy-sick like. I doubt it's the source for your nausea and occasional lower pain, though. But it certainly won't hurt anything if you can try some OTC Claritin and see if you can't get your drainage to stop. If you're like me, you are prone to sinus infections anyways, so stopping the drainage now (before everything starts to bloom) can help keep you from getting a full-blown sinus infection later. Needing antibiotics to get over one of those just makes your guts even worse.