We had that all the time with DD, and DH and I felt very alone dealing with it. GP was no help. At her 6 week checkup, Paed suggested she had reflux, not a problem, try and keep her upright 30 minutes after every feed but it will still happen at times and especially if she's upset - but it was not our fault, so just settle her again and give her another small feed if she needed it.
If there is wind trapped, babies can get very upset as it hurts and they can't make it move so well.
If they are greedy and eat too much, their tummy hurts because it is so small and is stretching.
If they are overtired or overexcited (sometimes too many visitors or too much exciting play at the wrong time) and just can't get to sleep, they get upset and can get so upset they go beyond being capable of settling for sleep until they literally run out of all energy.
There are so many reasons why it can happen, and you feel dreadful after each one. But it's part of them getting to know the world as much as anything else. And as long as you are caring for them, trying to find out and deal with potential problems, and just loving them, that's the important bit. And then they wake up after a good sleep and digesting the big feed and full of energy and smiles and make you feel so much better.