I have a basement toilet and shower that was in place when I bought the house. In the past couple of weeks, when the toilet is flushed, the contents back up into the shower drain about two feet away in the opposite direction of the drain flow. After a few seconds, the back up subsides. There is no problem with the drainage flow in the shower. Can someone tell me where to start investigating without getting an expensive plumber in? Obviously there must be an obstruction reducing the flow in the drain to the sewers. Can I do anything without removing the toilet? I would appreciate any advice at all.