GfCI trips in bathroom
GFCIs trip when the incoming "hot" current does not return on the neutral line. This situation could arise due to:


normal operation of an inductive device (like a motor, such as you would find in a fan or refrigerator). These devices shouldn't be on a GFCI.
A neutral / ground fault. This would be due to a defect in the wiring or an appliance.
An electric path to ground through something else, like a person. This is the primary purpose of a GFCI and the reason they're required in wet locations: to save your life!


An over-current condition should not cause the GFCI to trip. (Although it might trip the circuit breaker. Circuit breakers have entirely different purposes and detect a different category of problems.)

Note also that you can have a GFCI on a circuit without the entire circuit being protected by the GFCI outlet. Anything attached to the "load" side (e.g. downstream) of the GFCI outlet will be protected. Anything on the "line" side (e.g. upstream) will not affect the GFCI outlet.

To find out exactly where the problem is, it would be helpful to know if the GFCI outlet trips immediately after you reset it, when any device is turned on, or when a particular device is turned on.