She could have had an "off" day but it's more likely that something bothered her and that's not always easy to find. Some days my mare has some stomach trouble, or it's too hot and she's tired, and she lets me know that the saddle being cinched around her is NOT appreciated (but it's always tolerated). Other times it might be the bridle is off and someplace is pinchy. I've even gotten the saddle a bit too far forward and that made the crupper move up under her tail too much - that one she let me know in no uncertain terms! LOL
My point is my mare is a good driving horse and she tolerates whatever happens but there are days that things aren't just right. That could be this horse's trouble too. So I'd suggest another drive and maybe run you hands over her before tacking up to see if she touchy in some area.
If she acccepts the driving like my mare and you like the horse then I wouldn't worry too much about first impressions.
My point is my mare is a good driving horse and she tolerates whatever happens but there are days that things aren't just right. That could be this horse's trouble too. So I'd suggest another drive and maybe run you hands over her before tacking up to see if she touchy in some area.
If she acccepts the driving like my mare and you like the horse then I wouldn't worry too much about first impressions.