Bathroom light tripping GFCI?
People often get very confused by GFCI trips

We install ground fault detectors (that's RCD to you Brits), but then we're caught off guard when we actually catch one!  

If a GFCI repeatedly trips, then an appliance downstream or the wiring probably has a ground fault. 

Then it's a matter of divide and conquer. Break things out of the circuit until you find the culprit. 

My hunch is it's an old or cheap fluorescent ballast.  1000bulbs.com stocks a huge variety.