Why do we feel the need to poop when we feel the need to poop, and then if we don't and we wait it goes away and returns after a few minutes?

The intestines has muscular contractions to move the waste further down the digestive tract. The lower waste is contracted down as a result of the upper waste being contracted down. So when the upper waste is being contracted down it causes pressure on the lower waste, in result giving the feeling you need to potty. Saying that, this is not a steady process, the muscle contractions of digestion is intermittent (occurring at irregular intervals), in result, giving the idea that we need to go, but we now don't, but now we do again.

Why is this in DIY? To answer your question, I guess after holding it for so long your body realizes that you don't have to opportunity at the moment, and stops telling you what you need to do.

Gas