We have a washer and bathroom in basement... a month ago our washer drained and it came out shower. We have a septic pump and it quit working so we replaced the pump had the septic pumped and a plumber snaked the drains everything worked fine and tonight we were doing laundry and it overflowed again the pump is still working fine.

The blockage will be found downstream from the shower. I consider it poor plumbing practice to put a shower a washer on the same drain line. You have lint and grease of the washer and hair and grease from the shower. Be aware that your shower is the lowest points in his drainage system and will be the first to back up. Good luck, Tom.