Q: Car overheats, heater malfunction

Several days ago, the car started overheating. Since then, it has been happening after quite short driving distances. I've noticed that the heater is malfunctioning then as well. It also happened that I drive around town and the operating temperature is as it should be. However, when I go on the highway, the temperature would start increasing and the heater would work fine. After having the codes read, one appeared indicating a bad coolant temperature sensor. I had the coolant flushed the other day, and it seemed to me that the overheating was even worse while I was driving the car afterwards. It was as if I was driving a car with a manual and the braking was like when you still have the clutch engaged and you let go off the accelerator. What could be causing the overheating? Could this be because I haven't still replaced the ATF?