Bro..
I have seen this phenomenon happen with my Corolla-88 many years ago. It all depends on many factors:
- the load in your car
- time of the day i.e ambient temp
- engine condition
Your engine may have been weak already. Its like running a 100M dash race with no excercise. Thats why the temp kept on climibing as the engine was working more than its capacity (or at its limit) for about 25 min (50 km @ 120kph). This is a classic example of when "Horsepower (HHP) Test" is performed on engines. You just pushed it too hard.
Its a wrong concept that if you increase the speed, your temp will come down. This proposition works until you are at the upper limit of your engine loading. If you have weak engine with heavy load (passengers + luggage, engine not maintained or tuned properly, etc), increasing the speed will only do harm.
Final Words:
Always check your gauges when driving on highways. If the temp is going up, you may be pushing it the paddle tooo hard..!!!