Flexeril can cause stomach pain and nausea so it that is what you are experiencing, you may want to take another look at the medication.
Never heard about that association directly, BUT muscle relaxants are narcotics and as such slow digestion. A slower digestion may mean more pressure on the LES due to (a) large meals, and/or (b) pressure on the abdomen (due to exertion, belly fat, or both), which would, over time, weaken it.
You may do well simply to follow a GERD diet (especially the part about "smaller, more frequent meals"). You might also want to avoid eating fatty foods, as these both slow digestion and cause increased acid in the stomach.