Yes, a barium swallow is a simple way to see how the wrap is behaving, and thus determine if it's doing its job.
Since reflux is always possible after a Nissen--just less extreme an less often--sometimes people start getting some after overeating and drinking--especially during the first year. Sometimes the holidays are enough to set off a problem.
However, if you're not overindulging, it doesn't really make sense that you're having problems right now. You're doing the right thing to follow up with your doctor. He/she will be able to get an idea as to whether or not you need to go further in your search for the culprit.
This the other thing. The esophagus is a dumb organ, and it interprets all pain as reflux. If you injured or irritated your wrap area and esophagus, it could be mimicking reflux, and creating the same type of pain that you experienced pre-Nissen. If things were badly shaken up and stressed by the accident/seatbelt, they could be swollen and irritated.
Try not to worry, and don't assume you're in for another surgery. See your doc, maybe get a barium swallow, and allow things to heal. Be more careful about your eating and back off to more bland, soft, and easier to swallow choices. Hopefully with healing you can get back to normal.
Nissen Fundoplication 2/09
"Whatever you fight, you strengthen, and what you resist, persists.”
“Worry pretends to be necessary but serves no useful purpose”
“Accept - then act. Whatever the present moment contains, accept it as if you had chosen it. Always work with it, not against it.”