Kid Tueng คิดถึง litterally means "think towards" and can mean different levels of "I've been thinking of you". Ranging from "I thought of you the other day when I ate some spoiled food" to the much better "I miss you."
Just curious, does Thai girl say "I miss you" in English easily? I mean, when a girl say that to you, does she really understand the meaning of it? Or maybe the meaning is different if you translate it to Thai?