Any load on circuit trips bathroom breaker
It sounds like a wire may have been damaged during the install. I am guessing at this point that it is a gfci breaker since it only trips with a load. In this case I would be looking for a skinned neutral or a point in the box that the neutral and ground may now be touching. I don't believe it would be a hot wire because it holds with no load,  why did it start after months? Wires relaxing , home vibrations but at least this provides a location to look in the boxes on the neutral , it could be a staple in the wiring but I would check boxes first.