I nearly passed out - what happened!?

The past two days I have been getting headaches, but I thought that was because I've been sleeping too much lately. But I got a good amount of sleep last night so I woke up feeling fine. As I was having a shower, I started feeling really light headed. I felt like throwing up and I was swaying on the spot. Everything was really bright and I couldn't really see properly. My heart was thumping hard and I was really breathless. I thought it was just because of all the steam in the shower, so I got out, and I was so weak that I could barely put my clothes back on. I lay down and my hands and feet were tingling. It passed after about 15 minutes, but now I'm feeling really tired even though I woke up refreshed. Does anyone know why that happened? I recently became vegetarian about 2 weeks ago... Could that have anything to do with it?