If this started suddenly and you're sure it's not an issue of an incorrectly installed cooling system, make sure your cpu cooler fan is actually running. If you have a really silent one it might be that it has stopped working and you just didn't notice. If it runs, you could just reinstall it with fresh thermal paste.
Beyond that, if you're sure that you're just doing light tasks or its idling at this temperature, you might have malware that's pushing the load.
