Why is getting high the engine temperature on speeds above 100 km/h?

Hi I have a Ford Focus 2001 /1.6 (Automatic, Zetec SE) and I have noticed that in the last couple of days on the highway if I push it over 100 km/h the engine temperature is rising suddenly to maximum. If I drop the speed below 100 km/h the temp. Is dropping back to the safe zone.
I should mention that the cooling liquid is on the normal level, but I did find it one time on minimum (the first time when this rising temp occurred).
And I had changed the engine oil 1 month ago I am using now Castrol 5w30 A1
I am wondering is it the cooling system pipes (probably stacked somewhere ) or maybe the new oil I am using causes the overheating or the Thermostat sensor (temp sensor)
I live in Cyprus (35-40 degrees Celsius)
Thank you in advance.
Alex