I recently had a basement room remodeled, and I now have a drain problem.

The house is on a hillside, so the basement consists of 2 adjoining rooms on two different levels, the slab of the upper room is about 3 feet higher than the slab of the lower room.

All the plumbing in the basement drains to a septic tank.

Before the remodel, the lower room had a toilet, a laundry sink, and a standpipe drain for a washing machine, and a hot water heater. The upper room had a sink.

As remodeled, the water heater has been moved to the upper room, directly on the other side of the wall from its previous location. The washing machine has been moved to the upper room and uses the supply and drain from the sink formerly located there.

In the lower room, the toilet remains in its original place. A new shower is located where the washing machine standpipe had been. A new lavatory was installed in the location where the water heater once stood. The supply and drain for the laundry sink were capped off.

Everything seems to work fine --- except for one thing. Whenever the washing machine drains, I get a "glub glub glub" sound from all three fixtures in the lower room. An air bubble comes up in the toilet. And about a gallon of water from the washer backs up in the shower. It drains fine after that.

All the fixtures except the new lavatory are hooked up exactly where old fixtures were. (Of course, the shower drain is open at floor level instead of 3' above, when it was the laundry drain.)

What's going on?

I have very little confidence in my contractor to fix this, but I want to diagnose the problem and hire another plumber. I also want to know where the contractor went wrong.