I passed out. Do you know exactly why?

I woke up and ate an omelet and drank a cup off coffee then went to school. I had really bad cramps and was on my heavy period. I had horrible cramps since I got to school and when I was in the class right before lunch I kinda felt lightheaded and dizzy. And my eyes were shutting as if I were going to sleep. Then I managed to stay awake for that class and then it was lunch time and when everyone was standing In line to go to lunch I started feeling lightheaded again and I had a cold and hot sensation in my head. I was sweating and shaking and I couldn't see clearly then all I knew was I passed out for like 10 secs. And my friends picked me up and took me to a teacher. And they took me to a nurse and the nurse gave me something to eat and I felt better. Why did I pass out? This isn't the first time I did it, I'm 13 and in my whole lifetime I probably passed out on an average of 5 times.