why am i responsible to fix the waterproof of my bathroom floor?

Hi all,

Anyone can help me? I bought a 13 year old condo 2 years ago. Recently, the management requested me to open my door to let a contractor to come in to my condo to inspect my bathroom - supposingly the unit below me has very bad water leakage on the ceiling of their bathroom.....the contracttor claimed that the waterproof of my floor is no good, causing the leak & damaging their ceiling. They demanded for me to fix the damage - 1st - at my own cost, then when i probed the management staff for proof of why am i responsible to fix it, she gave me a 'supplementary agreement' signed between the developer & the management company...then she told me the cost is to be shared with the unit owner below me...we are supposed to contact each other to solve this case. I am not sure why am i the one responsible to fix the floor? its their ceiling, they can fix it at their end because my bathroom is totally fine without any problem.
May i know if this is the correct law guarding the condo owners in malaysia? The owner above must fix the floor? Share cost? Or it should be under management as its not inside my unit but betweeen my unit & the unit below. I thought waterproof should be the responsibility of management?

Anyone - pls advice. Thanks.