I have a two showers upstairs and it just started leaking when the shower is running it will pour water out of the downstairs drain that the washer drains out of. What could this be

You have blockage between the washer stand pipe and the main drain line. If you have laundry tub close to washer stand try snaking line from there to clear.

Harold nailed it! The blockage's downstream from the washer. You may snake from the washer stand pipe or from the washer roof vent. If you snake from the roof put out enough cable to reach the base and 15' more. Good luck, Tom