it can make them feel fizzy, but i know off many mares who would have bad bucking fits in the spring, not because of the grass or due to 'nastiness' but because they came back into season and they had very sore backs due to it.  one friend actually got her mare a sheepskin pad for under her saddle which helped the horse, and although she would throw in the odd buck at the beginning of her first season, it did reduce some of her discomfort...and that of the rider as she kept getting flung off!  however, it has not helped with every mare, but it's a suggestion.  does she look in pain at all, or does she have a more cheeky face on her, lol! xx

Quite possibly, but have you had the fit of her saddle checked recently? With the spring grass come extra calories and generally speaking weight gain so it could be that the sadlle isn't quite fitting right and needs adjusting. If that doesn't help it would be worth getting her back checked.