Why did my heart rate shoot up to 199 during a moderate training run.?

Toner · Yahoo User

I am female, 57 years old and a marathon runner. This morning I went out for a 10 mile run at 4:30 am. It was 81 degrees, 86% humidity and no breeze. At the turn around point I felt my heart rate surge and looked at my HR monitor. It read 199. My average heart rate when I run is between 136 - 143. I immediately started walking, put my arms up, drank water (I stay well hydrated before, during and after my runs). After 10 minutes it had come down to 162. I then sat down and it went almost immediately down to 80. I continued to run a slow pace back to my car. During that time I got a throbbing in my left temple. I seem fine now but this is upsetting me. I have been under some stress but running usually calms me down. What happened to me out there? My average pace by the way is approximately 13:00 miles.