Well, something had to happen. You just need to figure out what it was. You most likely (if your description is correct) have a fault to ground from the neutral wire. Could be caused by abrasion such as a neutral wire pinched between a junction box and a paddle fan mounting plate that finally wore through.

BUT, are you sure that is a GFCI breaker, or is it an AFCI breaker? Either is possible. An older house without a ground wire throughout the circuit might have a GFCI. A newer house would have an AFCI.

as Fred said, determine if this is a regular breaker, a GFCI or an AFCI. GFCI breaker can trip for two reasons (this only applies to breakers not GFCI outlets). One is overload but that doesn't appear to be the issue if you haven't added any new loads. The other is you have a ground fault which has nothing to do with load. Seeing as operating a light switch triggered the issue, I would examine the wires to the switch if it's a walls with , the wires at the light fixture if it is permanently wired or the outlet if it's a plug in. Make sure all wires are tight. Could be the lamp itself as well. If that doesn't work you will need to check every receptacle on the circuit . It's not likely behind a wall but rather a connection

3 reasons. Short circuit, overload, and ground fault. The only way it could be a short circuit from the description is if it was also an arc fault. But since it happens with both the office light and a lamp plugged into any outlet, the chances of both lamps having arc faults at one time is very slim. It also makes the likely hood of the fault being in the switch leg no greater than anywhere else. Since you don't have to flip a switch to make it trip, but it will trip if you plug in a lamp, then it isn't the hot wire, or a short circuit. Otherwise, the breaker would trip without plugging anything in. Since it only trips when there is a load, that rules out a short circuit in the house wiring on the hot side, but brings the neutral wire into the picture. The neutral wire is tied together throughout the circuit, but some of the current isn't getting back to the breaker. This makes the GFCI unbalanced and trips it. Since the neutral is continuous throughout the circuit, there is no way to assume that the fault is occurring in any particular place on the circuit. It could be due to a nail in the wall from a new picture, someone tying in neutrals from another circuit when changing a switch or light fixture, or even a newly developed neutral to ground short inside a permanently installed fixture whether it is turned on at the time or not. You don't have to check every receptacle. That won't tell you anything unless you know what you are looking for and get really lucky. It also won't tell you if the problem happens to be inside the wall.

Start by pulling a receptacle that is assumed to be in the middle of the circuit. Disconnect and separate both the neutral wires and hot wires from the receptacle. (This doesn't work if there is only one cable in the box and all the receptacles are fed individually from the ceiling junction box.) Turn the breaker on and test again with the lamp, noting which outlets are still hot and which are not. If the breaker still trips when you plug into a hot receptacle, the problem is in the half of the circuit that is still hot. If not, it is in the other half. Split the half of the circuit that the fault is in in half again, and repeat testing. Do this until you have narrowed the problem down to one cable going to one box, and don't forget to include any fixed lighting in your search. This should only need to be done three or four times instead of every receptacle. Once you narrow it down to one junction box or wire leading to that junction box, look for signs of wear on the wires in that box, particularly the white ones. Once you have disconnected the light fixture wires, and made sure none of the leads in the box are touching each other or the box, test with the plug in lamp again. If it still trips, loosen any cable clamp completely and try again. If it still trips, then there is something wrong with the wire in the wall feeding that box. But it is most likely what I said to begin with. So, the easiest and quickest thing to do would be to pull the switch going to the light and disconnect it and separate all the neutrals and hot wires in that box first and do a test. (This only works if the light is fed from the switch and not the switch fed from the light's junction box.) You can probably get lucky and save yourself a little time by starting to divide the circuit at the switches.