Good suggestions from forumers but just ask management/maintenance to send guy to see. They can tell immediately what is the issue instead of you guessing here. Nobody really know. It's FOC so do this step first ba. They will also tell you it's your responsibility to fix or downstairs have to fix it themselves. If it's your responsibility then get a trusted plumber to go see and do the job, I can share with you plumber's contact if you want, fixed the condo leaking at my place before. Usually it just cost few hundred to abt 1k. If u need to hack and redo tiles , which i also did before, workmanship for Chinese is abt 2.6k for normal size toilet, tiles u buy yourself. If indon can be slightly cheaper gua.