Sorry to hear you're not feeling well. I can't remember how far out you are after your surgery, but unless you're in the very early stages, I can't say that the symptoms you're experiencing are related to your Nissen.
You could be getting a virus. Just a reminder...if you do get a stomach bug, be sure to have anti nausea drugs on hand, so you can stop the nausea from causing you to vomit. Vomiting can create real problems with a newly wrapped stomach.
I hope it's nothing, and you feel better soon.