Did you fall against something? Is there a bruise? We can't see you or examine you, so we don't know why either.

What does your doctor say?

Muscle strain, stomach ulcer, bowel problems, who knows?
Doctors tend to say the most common first, strain, unless you have multiple symptoms, so they may say 'soak in the tub and put a heating pad on it and call me in a few days if it doesn't get better.'
Your torso has all sorts of organs in it. Pain can refer from somewhere else. It's just too difficult to diagnose easily.
Doctors are not miracle diagnosticians either. I spent over 30 years getting a pain diagnosed properly - finally, by a chiropractor - for pain not in my back.