SUDDEN Increase in Temperature, need help!

[jetuomi]

Hey all, just friday night, I'm cruising around, and my temperature gauge is at the left-side of the O in NORMAL (where it hasn't moved from for 4 months now!, and I haven't made any engine mods for at least 3 weeks now)

All of a sudden, it jumps past R, and almost past the M, to sit between M and A..

And now, it warms up in 5 minutes, right back upto to between M and A..
(and it usually never warms up that quick)

Finally, I've checked the oil, which is fine, coolant level looks fine, thermostat works (the upper rad hose is hot!)

My question is, WHAT ELSE COULD IT BE?????
And, should it worry ME? I just bought a 180* thermostat, and I'm going to drain, flush, and refill the rad..

Thanks guys!