Why did I faint after an HPV and whooping cough shot?

I'm not afraid of needles or blood or anything like that. If it changes anything I'm a guy. I was just standing there while my mom made an appointment for my second HPV shot and I started to feel kind of dizzy and light headed. I didnt do anything about it because that happenes to me sometimes at school. I saw like all white and my eyes felt all weird but then I felt normal. The last thing that happened is I fell while i was trying to sit down but I felt like I was consious while I was falling. I hear a nurse say get jayson (thats my doctor) and he helped me up, got me water and made me lay down for like 5 minutes, sit up for 5 and then move to a chair and sit for 5 minutes. None of that probly matters for the answer. All I want to know is why I fainted