I have had the same thing happen. I was riding along the road when three horses in the near pasture come running up. One was a mini stud, the others where full grown paint / quarter crosses. The mini started to at stupid and my mare was not in heat but erritated. The other two horses came up and took off causing my paint mare to bolt. I quickly got her to slow down in a circle, but she kept spinning in circles and ended up in the road. I was set in the saddle good but there was a van coming and my mare was not paying attention to me, she kept going in circles. The guy almost hit my horse. Luckly the mare and I end up on the other side, but she was very wound up and started acting like she wanted to go go go go. I fought her the entire way home to keep her slowed down, 2 mile ride home was very exhausting. I never took her out again except just around the yard, pasture, or the trails behind the house that had no other horses around. The one rein stop saved us, but still could have been bad since the mare was acting hot blooded.