The kind of rude behaviour that infuriates me

It occured when I was a high-school student. After school, I would get together at my friend's home off and on. It was one of the days that I went to his house. There were my friend, my his mother, me and his dog then. When we were playing the computer game, his mother came in the room, and started pouring milk into cups. After filling two cups with milk, she told me "Woojin it is left only one more cup of milk. I'm so sorry, this is for my pet." And she gave milk to the dog. Also, they drank a cup of milk each. I couldn't understand the situation. Is it possible to happen to a guest? I had known them for a long time and, I was pretty sure that they didn't dislike me. I don't know.... I can't give a picture of the feeling that I felt then. Despite of everything, I felt suck!!!!!!!