Can someone help me about the Philippine law regarding real estate?

I bought a real estate property in the Philippines several years ago after checking it with the provincial land registration, to be certain that it had no attachments and that it is "clean". I have a certified true copy of the previous title as well as the owner's copy of the new title under my name. When I started construction years after, someone suddenly appeared claiming that the land is her inheritance, though she does not have any title to prove it, she has a copy of an allegedly extra-judicial partition / agreement under her deceased parents names. Please help me what to do and how not to lose what I have worked hard for so many years. Thanks.