Weirdest Thing...Help?

Okay so I had today off and I have been reading for a couple hours. I had a knock at the door and it was a couple of friends so I was standing outside talking to them. About 10 min. into the convo I all of a sudden felt sick to my stomach like I was going to through up right there so I asked them if they could come back later because I wasn't feeling well and when I turned to walk in the door I passed out. I woke up sitting against the chair right by my front door with them saying "Are you okay?" I closed the door and had to sit there for a minute because my vision was still kind of blacked out and fuzzy and my ears were ringing. Once that stopped I got up and I felt fairly normal again. Why did this happen?? I've only had that happen one other time and it was because I was at a track meet in 105 degree weather and I had just ran a long hard race and I was standing on the podium but nothing like that was happening today? What happened to me?