So a couple of days ago while I was driving, the radiator fans suddenly came on (they don't usually come on if I haven't driven for a long time), and the temperature gauge shot up to more than 60% (it usually stays below 30%). Then after some more driving, while parking it shot up all the way to H before coming down again in a few seconds.

The next morning when the engine cooled down I checked the reservoir level and it was below min (note: I opened the radiator cap as well, but I could see some metal right beneath the cap, but no sign of any liquid in the radiator). This is the first time I've opened the radiator cap so I'm not sure what to expect.

So I bought some coolant and just topped off the reservoir coz thats what they said on the internet. But the temperature continues to spike if I drive for some time, and then go down back to 40% or something. The radiator fans are constantly whirring now. Starting the A/C or anything does not change anything. When I open the hood after driving I get a bad small, like something is rotten or something like that....

Are these peculiar symptoms of anything? Do I need to flush the coolant entirely before filling a new coolant? The car is a 03 Sentra, 62k miles on it