How do you know when you are full?

How does one know when they are full?  I have seen this question asked before but I still am unclear on what to look for.

How is it possible that I can go from eating my tiny bites, chewing a billion times, pausing between bites, to taking ONE MORE BITE and winding up in extreme PAIN????  (Even a H-rsh-y's kiss as dessert could send me to the painful hard stop!!!!)

One more thing.....why oh why do I (we bandsters) have burps from DEEP DOWN inside?  Multiple burps?  Can anyone explain??