Three things come to mind:
1. Bad pressure switch - It's function is to shut the pump off when the pressure builds to a pre-set level.
2. Leak or open valve- Even a small leak could cause the pressure to fall short of that required to trip the pressure switch. Make sure all your faucets and valves are closed, including your outdoor shower. Your toilet could also be leaking.
3. Empty tank - Hopefully you've checked your tank but if it's almost empty, the pump will run and run. Your water flow will be low as well.
Here's a link to a more exhaustive list: