It overheated again?

Ok sometime this summer I over heated my car. So I pulled over let it cool off and then went home to fill it with more water. I made sure the car was running while I filled it with water and also flushed it. I didnt drive it until my mom used it a couple days later and she said it over heated again after she made it to town. What could be wrong what should I do? Will it be expensive to fix?