When you're painting outside, quick changes in light and sudden shifts in weather are all part of the game. A good tip is to check your weather app before you head out. It'll give you a heads-up if it's going to rain or be too windy, saving you from a wasted trip.
And about the changing light, always note the exact time you start painting, if you attempt to paint on another day, the time reference will help you find similar conditions. A hack for the shifting light is to make a quick sketch of what you're about to paint and snap a picture too. That way, even if the light changes, you've got a reference to help you finish your watercolor masterpiece.