toilet issue need help

Today when i woke up i used the toilet upstairs flushed, didn't hear the "flush" (which is an issue in and of itself but nothing major just didn't hold the lever down long enough) came downstairs to my office a few minutes later nature called again, went to the bathroom downstairs. as i was going, i heard what i can best describe as a "crashing whooshing" sound followed by sounds of water moving through the pipes from the upstairs toilet. when i finished i got up turned around to flush saw a "paint bubble" coming from the wall that was filled with water. first thing i did was "pop" the bubble and drain it. then i peeled the paint back to allow the area to dry. i went upstairs after that to investigate then saw a bunch of water on the floor (centering from the toilet) cleaned that up. my question now goes to how did this happen and what can i do to investigate if it is something "easy" for me to fix or if i need to hire a plumber to take care of it? one thing i found out as of typing this when my fiance went to the bathroom last night, she said that it clogged on her after using it. could it just be that without her telling me that piece of information before hand that the pressure from my flush on top of "the clog" caused it to push through the drain? any and all help / advice is welcome and thank you in advance.
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