Are you on any antibiotic for the sinus infection, as that might cause it. It is possible that you are having some sinus drainage (phlegm) down your throat and this could be getting into your stomach and upsetting it. As far as it causing real pain, I'm not sure, but it can cause it to be upset.
Hope you feel better...JJ....
When you come to the end of your rope..tie a knot and hang on!