Light headed

So yesterday morning I gave blood when the blood mobile came to my work. I knew that I hadn't been at the gym Saturday and Sunday so I packed up and headed there. For some weird reason I was dreading it. I just didn't feel like being there. I get going on the treadmill. When I hit about 1 mile I get lightheaded but it passes fast. I have 2 more occurances before I finally say ok enoughs enough and get off. I only did 1.7...

I realized it was because I gave blood how stupid was I to go run after that. Uggh /fail at the gym.