Calling all experienced moms

My little guy has a problem. Whenever Josiah has a bm his tummy hurts. He cries and doubles over in pain. His stool is soft, he is not constipated. This has been going on about a month now. No new foods have been introduced and after the bm he is fine. He doesn't complain about a tummy ache at any other time except during a bm. And it is horrible to watch him in so much pain. He literally takes his hand and jabs it into his tummy trying to get the pain to go away. HELP!