You are being threatened by the one of your apartment owners alleging you of the acquisition of your flat illegally. However, instead of being perturbed by such threatening, you need to check up your papers and at the same time, go to the panchayat office and ensure that there has been compliance of their prevalent in the process of registration of your flats. Even in the past, there had been several issues of litigation in the panchayat areas of Hyderabad.
Now I would like to know whether you have acquired this flat with a bank- loan or the same has been acquired through your savings. In case, the loan has been sanctioned through the loan facility of the bank, they would definitely go through all your property papers while advancing the loan amount.
For your own safety, you may approach a lawyer to resolve your issues and you may take up steps accordingly.