When I flushed my downstairs toilet, it sucked the water from the upstairs toilet and created a brief "fountain" in the upstairs toilet. Then when I flushed the upstairs toilet, there was a loud "draining" sound from under the upstairs bathtub. Nothing strange occured in the downstairs toilet. *The upstairs toilet is basically right above the downstairs toilet. This is the first time anything like this has happened in the 6 years that we have lived in the house. Recently we had the bathtubs drain pipe replaced.
What's going on here?
Thanks in advance for your tips/advice/wisdom.