Yamaha Nytro Overheating

I have a 2006 Yamaha Nytro in great shape with about 2,500 miles. I was teaching my daughter to drive this weekend in about 10 degree weather but after driving between 10 and 15 mph the overheat light came on. I checked the fluid and it was full and very hot. When we hit an opening I sped up and it soon cleared the warning light but it soon came back on when we slowed down again. This happened several times as soon as I drove slow for about 10 minutes or so. Is this normal or should I do something?

Thanks in advance for your help

Yamaha Nytro Overheating