Leakage issue

I stay in a old building age more than 35 years , its a 4 floor bilding and my flat is in 3rd floor. I bought this falt in 2012 , before entering the flat , the flat owner below my flat told me he have leakage issue, same was addressed along with renovation of my flat by puting substantial amount in changing internal pipes and oter fittings. Subsequely he requested to paint his house also which i denied and told him to approach previous owner. Issue was closed then. In 2016 again he complained and subsquently he concluded that its exteranl one & matter was closed. Now in last mansoon the 2 flats below him complained to him fo laekage he ( flat owner below mine ) started saying its all due to me , subsequently the flat onwer come along with a plumber to my flat suggested for some changes , same was done and confirmed ..no response was given. Subsequenly the flat owner below did some repair work including plumbing and started complaining leakage , also started pointed to a nahanitrap which connects to the may darainage with external darinage pipes . Cause of this damage could be due to 1. Internal issue in my flat. 2. Damage caused by below flat due to repair. 3. againg building . When i dicsussed with plumber , plumber agreed that there is no damage in my bath room and is not the cause of such leakage. the issue could be due to other 2 points. It is a recurring issue and now i am fedup and dont want to attend as i feel its not my fault. Please advise with my obligation and responsibility ... leakage is common and is issue in all most all flat as the building is very old.