Bad Buttermilk

How can you tell if buttermilk has gone bad? I mean besides using it in cooking and then ending up half the night with roiling innards. It did smell a little bit sour — but then, it IS buttermilk. I used it on the 4th and the pull date was the 6th and to my knowledge it had been properly refrigerated. I’m about 99% certain that the buttermilk caused my digestive woes.