If you are on municipal water (not a well), then what you suggested is certainly possible. Happened to me once last year. I popped the hose in the spa and went off to do something for a few minutes, and came back to a hundred gallons of brown water. Turns out the city was flushing the water mains in the area, which stirs up iron and temporarily discolors the water. I drained that water, and refilled when the water ran clear again.
Brewman