A menopausal woman who is mesuleket damim (halachically not expected to menstruate) is no longer required to observe onot perishah – unless she resumes menstruating three times.
According to some opinions a peri/menopausal woman attains the halachic status of a mesuleket damim if she does not menstruate for at least 90 days after the age of 53. Other opinions set later ages.
Since you started bleeding on the 90th day, you are required to observe this interval, even if you are over 53. (Had you begun bleeding on the 91st day, you would have been required to observe the interval only if you are younger than 53.)
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B'Hatzlacha!