Amid speculation that the real reason Hillary Clinton canceled her trip with Bill to Central America was that she is furious with him, sources in Washington say Secret Service agents are vouching for the account that she aggravated an old back injury while skiing in Utah. According to officials, Mrs. Clinton was sideswiped by another skier, which led to a review by Secret Service headquarters to make sure her security detail had been in place to deal with any sudden, threatening movement toward the First Lady. Mrs. Clinton, who wore a hat and dark glasses, apparently went unrecognized by the crowd at Deer Valley. She was standing at the bottom of a slope filled with inexperienced skiers when one of them lost control and ran over the back of her skis. Hillary fell, injuring her back in the process; she complained later of being in pain. (The Secret Service found nothing suspect about the collision.) Marsha Berry, Mrs. Clinton’s spokeswoman, has insisted all along that Hillary stayed home to recover for her forthcoming trip with Chelsea to Africa. And no, they won’t be joined by Bill.