Ping out? What am I doing wrong?

Recently, I have been extremely light headed while doing yard work and any strenuous indoor activity. I eat, so I don't think that's the problem. I'm 20, 5'2" and I weigh about 99-100 pounds on most days. I ate breakfast about three hours ago and I did yard work for twenty minutes before I had to sit down for fear of ping out. I got to the point of seeins in my vision, but I didn't actually p out. What am I doing wrong? I ate breakfast and I'm always extremely hydrated. I drink 50-60 ounces of water a day, normally. Any clues as to what could be wrong with me? Should I see a doctor about it? Thanks for any help. :/