What could cause this kind of leak?

There is a small leak in my bathroom ceiling directly over the tank of my toilet. The water is brown/rusty in color. It creates a puddle on the tank of my toilet. It has happened three times within the past month so far. the first two times was just a dried light brown spot, but the last time I saw a wet puddle. I... show more There is a small leak in my bathroom ceiling directly over the tank of my toilet. The water is brown/rusty in color. It creates a puddle on the tank of my toilet. It has happened three times within the past month so far. the first two times was just a dried light brown spot, but the last time I saw a wet puddle. I live on the first floor of a two family house. What does it sound like could be wrong and what are possible causes of the source of the leak?
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