Can weight gain after tummy tuck cause abdominal pain?

I don't think 5-10 lb weight gain would cause abdominal pain. That's not much weight and your tissues will stretch a bit. There is likely something else going on. Endoscopy sounds like a good plan!

If you have gained 15-20 pounds since your tummy tuck, it is entirely possible that your symptoms are made worse by the increased pressure on your newly tightened abdominal wall.  Checking everything out with an endoscopy is a good idea.  The best test would be to lose some of that weight back and see how you feel.  

Usually a year after tummy tuck the pain should be gone. During the procedure the muscles are tightened and can cause a pressure or tight feeling post operatively.  It  is always best to follow up with your surgeon  to make sure everything is healing properly.