What would cause me to have stomach pain for three days?

Not enough data: Lots of possibilities. Will need more information about the pain, location, severity, inciting factors, relieving factors etc.

Many causes: You don't say where the pain is, or how severe. This could be anything from a flu bug (gastroenteritis), to acid reflux, to gallbladder problems, or even a bladder or vaginal infection. You should see a doctor since it is persisting. Good luck!