U.S. oil prices had their worst day since March, falling to their second-lowest level of the year after U.S. data showed an unexpected increase in oil stockpiles. U.S. crude inventories rose by nearly 3.3 million barrels last week, according to the U.S. Energy Information Administration—the first weekly increase since late March. Analysts and traders surveyed by The Wall Street Journal were anticipating a 3.5 million barrel decrease…