We had a plumber come out and unclog a drain in the bathroom sink on the main floor of our rambler house. There were no other clogged drains in our house. A few hours after the plumber left, I pulled the shower curtain back and I discovered our bathtub was full of black water and some kind of black sediment or something. We noticed that when running the water in the sink, the water was backing up into the bat tub. What is happening? Can we fix this or do we need a plumber again?