I am 35 yrs old. I have been having constant nausea for a few weeks. I had a sinus infection and know that the drainage can cause nausea but each day the nausea gets worse and I sometimes can't sleep because of it. How do I know when it is serious enough to see the doctor? Loss of appetite.

It's serious enough to see a doctor because of the length of time you've felt it.
(I always ask this though - are you drinking enough fluids despite loss of appetite?)

Probably not. I try to. I don't drink soda. I am a water drinker. Lately I can only get through half a cup of my morning coffee. I can barely get down more than a few bites of food each meal. I am eating lots of crackers to keep me from vomiting.

I would say it's definitely time to see the doctor. This has been going on for weeks. You're obviously not getting better on your own, so it's time for some medical help to get you back to good health.