Hopefully I can convey my question properly. My townhome is about 20 yrs old (relatively new). It is two stories tall. I have 2 toilets upstairs and one toilet downstairs. This issue just started happening a few days ago. When I flush one of the upstairs toilets, it causes my downstairs toilet to "flush". I put flush in quotes because the water from the tank doesn't come down in this case but it seems the outlet from the bowl bubbles and the water leaves the bowl. The downstairs bowl is empty whenever the upstairs toilets are flushed.
I've also tried to flush the downstairs toilet by itself and even though things get flushed into the sewer, it doesn't fill the bowl entirely with water afterwards, even if I hold the handle down to let more water get into the bowl. It seems to be filled 60-70% of normal when the downstairs toilet is flushed by itself.
It doesn't seem like a blockage since nothing thankfully is coming back up. I don't know how flushing upstairs is causing the downstairs toilet to lose water from the bowl and causing it to not fill to its old level.
What could be causing this problem and would this be something I could fix myself or is this something more complex like re-piping the whole toilet plumbing system in the house?