What Causes Nausea After Running Long Distance?

I was feeling nauseous periodically during a 7 mile run. It would come and go until I had to stop and walk the last half mile. I have run for 30 years and in excellent shape for a 60 yr old male. I then ate a light meal, took a shower and set down to rest. I was overcome with sweating and then woke up on the floor after fainting for a few seconds. I laid down and the sweating stopped within a minute. No problems since then.