Jose,
Under normal operating conditions your temp gauge should be pointing straight up, which is usually right where you are describing... between the R and the M.
So, IMHO there is nothing wrong with your cooling system if that's where your temp gauge reads after the engine is warmed up.
Why your gauge read below the N before your trip is a mystery to me. I would say that your gauge was not working at all if it was reading below the N on a warmed up engine. Maybe there was a temporary or intermittent contact somewhere in the circuit or the gauge which caused it not to work.
At any rate, a temp gauge that reads in the R to M range on a warmed up engine is just fine.