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I've seen so many people, Including me, that when they start running for a couple of minutes, the sides of their waist hurt. I've heard that it's because they drank water before running and I've also heard that it's because they haven't drunk water during or before running. I think that may also be due to bad breathing, but I'm not sure.
This doesn't happen every time I start running. For instance, I do workouts 3 times a week and sometimes this happens to me once or twice a week.
As per LarissaGodzilla's comment, this is likely an exercise related transient abdominal pain, commonly known as a "Side stitch". Surprisingly enough, no one knows for certain why they happen, but the Wikipedia article linked above gives some advice for how to avoid them.
Also: What causes side stitches? and What to do against side stitch? on this site.
