Could be condensation on your exhaust ducting (warm air from your bathroom hits cold air in the attic). The ducting should be insulated (if it's not, I'd start there) and there should be a bend in the ducting that will allow water to collect and evaporate on the inside of the ducting versus just dripping down onto your ceiling.
As for who you'd call - I paid an electrician to install a new fan in my bathroom and he added the insulated ducting while he was up there.
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