Sympathy and all commiseration, Andy.
It has happened to me a few times, mostly with water, but sometimes solid objects. The boxfull of ball bearings was worst, as the impact was bad at 100kph, but even worse was trying to stay upright after. My back was sore for days, but it apparently provided amusement for the carload of uni-aged youth who tossed it.
Glad you weren't injured and did not fall, Andy. Startlinf, for sure and certainly unpleasant. I'm sorry.
All the best,
Dan.