Are you using stock cooling, or something aftermarket?
Either way 65C is perfectly safe and no cause for concern really. It's a bit warm for stock speeds, but if you only have stock cooling for example then it's to be expected.
When you say you left your PC idling, after 2-3 minutes of being idle windows starts doing background tasks that it doesn't want to do while your active using the PC. These tend to use quite a bit of CPU time when they run (which is why it doesn't do them while the PC is in use!) so the reality is your 'idle' PC has likely been very busy while you been away from it, explaining why the temps have spiked up.