I was laughing hard, and a few minutes later a knot appeared in my upper left stomach. it hurts really bad. what could it be?

Muscle spasm: Most likely it is a spasm of your abdominal muscles, also known as a charlie horse. It is common. People always think a lump is a hernia, and it is essentially unheard of to have a hernia in this location unless there was a previous surgical incision there. If it doesn't get better, see your doctor to be sure. Otherwise, keep laughing - it's good for the soul.