You sound like you might have a similar case to mine, although my hernia became paraesophageal due to an accident with the seat belt shoving the stomach. If you have sudden severe stomach or chest pain you must get to a hospital with the speed of lightning because they have about
a half hour to unkink your stomach to save your life. I had to wait for 4 1/2 months to get mine operated and worried the whole time because I could feel it trying to flip to the other side of my chest when I rolled over at night.
As for the spasms- it sounds like possibly a food trigger/allergy/sensitivity. You don't have a bunch of time, but you need to keep a food log and try to figure out if you have food triggers. I had the same symptoms, also relieved with burping most of the time. What you are doing by forcing a burp is "Valsalva maneuver." You can also try to get relief by coughing.
My trigger foods were a long list of seemingly unrelated foods: wine and beer, maple syrup, breads, cereals especially with dried fruits, dried fruits, sodas,
cookies, gelatin, guacamole, onions and garlic ... On searching I found out that they all contain sulfites. By avoiding sulfite foods I can keep my refluxing under control. You might research food allergies and see if any of the lists rings a bell.
My stomach spasms often triggered tachycardia from coronary artery spasms. Eventually it caused a heart attack when the spasms caused a tiny point of plaque to rupture.
If you do have food triggers, they won't go away when you have the fundoplication. You won't reflux food up into your esophagus, but the stomach will still jump around.