Why is my cardiovascular endurance so bad?

I've always had issues with cardio. I'm otherwise in pretty good shape. I'm no athlete, but I lift weights, I like yoga. I eat pretty well, and I'm vegetarian so I don't think it would be a cholestorol problem. I'm 21% body fat, so in a healthy range. I've never smoked a day in my life. But I just cannot seem to handle cardiovascular activity, particularily running. For example, we were running out on the track in gym today and I could barely finish jogging 2 laps, so 1/2 a mile. After that, I felt like I was going to pass out. I was disoriented and couldn't walk straight, my chest was aching because my heart was racing so fast, i had a headache, my speech was slurred.... and this was for at least 20 minutes after. I've always thought that I just need to excercise more, but is it possible that something else could be going on? If not, what would be a a good way to get some cardiovascular activity?