Why does it hurt to laugh?

i was telling two of my friends some jokes the other day, and we were all laughing. one of my friends was holding her belly while laughing, like when you have a stomach ache or gastric. she was looking in pain but happy at the same time (it was really sad you know). my other friend said it was because she had stitches. can someone explain to me what happened or what i should do? i mean like, will it heal? she was already like that around the end of last year.