No way do car instruments accurately read to a degree or three so the question is "why has the reading changed?"
Its unlikely that the engine is producing more heat assuming its running OK, so the issue is why is the cooling system less effective. Could be any number of things but best start with the simple like:
is the radiator clear or have you ingested a plastic bag
is the fan running? what is the cut in set for and should it have triggered?
has the water system leaked and the level is now low?
thermostat failed part closed?
rubber hose part collapsed?
fan belt slipping?
there really is no alternative but to potter round checking bits in order of simplest first.
