I notice you are taking Imitrex PRN. You did not mention whether you are still having migraine attacks. Migraines have been known to trigger bouts of nausea. Even severe stress headaches can trigger bouts of nausea. It may be of value to see if you are having concurrent headaches/migraines when your nausea hits. You may also wish to see whether certain food groups tend to trigger nausea. It is also true that medications can cease effectiveness, and cause a need to change medications. If you had been on NSAIDS for a long time, you could have caused some gastritis, and this too, combined with GERD, can also be expressed by the body as pain or nausea. Ask your primary doctor, or your gastroenterologist to consider switching your medication, and/or work you up for GERD (GastroEsophageal Reflux Disease).
Wishing you the best of health!