Why do I feel like I am going to pass out after exercising?

I hope to get some replies. Yes, I just registered to pose this question. I will try to keep it as simple as possible. I started exercising several months ago to get rid of excess weight I gained while being laid off. I chose an exercise bike and lost 40 lbs. in a couple of months. Went from 205 to 165. I didn't get light headed or anything then, and I pushed myself. Then I moved on to other forms of exercise, like weight lifting and running. I was fine with those too, but now I am having trouble with all of them even after only a little exertion. I was running 2.5 miles per run, but now can only muster 1 and that is pushing it. I started to work my legs today, but after about 15 minutes I felt like I was going to pass out and was breathing heavily. Any ideas as to why this might happen out of the blue? I don't think it has to do with nutrition, I cook 90%+/- of my meals from scratch and I don't drink anything except filtered water. Would appreciate help.

Edit: I didn't mean I registered to pose this question only. I meant, I did register today so that I could pose this question.