Hi alfaro,
The temperature of an idle PC will slowly increase a little bit over time. This is because the case and all components eventually heat up to the temperature of the air inside the case.
The inside air of the case is warmer than the outside air, (unless you have water cooling etc) and when the CPU fan and GPU fan blows warm air out (into the case), it contributes to the system temperature, this includes screws, motherboard, hard drives everything warms up. Some components more than other because they also generate their own heat or they are right next to a hot component.
After a few hours all of components, event the ones that don't generate heat own their own warmed up to a temperature because they absorb some of the heat. At this point the air in the box will become a bit more than a few hours before.
Maybe Monitor it for another 5 hours and see if it increases further. 33C is hardly hot, I seen idle PCs over 50C. It is also a good idea to "sleep" the computer when not used to save power.
The temperature should eventually level out, but of course the temperature also depends on the temperature in your room, and if colder air is sucked into the case then you will have a cooler GPU.
Gabor