Gas prices do fluctuate, often rising or falling significantly in a short period of time. Reasons for these fluctuations include supply and demand, infrastructure, and inflation.
Supply and demand are the main reasons why gas prices change so often. If only a limited supply of gas is available, prices rise because there is still a demand for gasoline. The opposite is also true.
Infrastructure can also have a bearing on the cost. Natural disasters or other interruptions of production interfere with the supply of gasoline that is available to the public, usually causing prices to spike noticeably.
Inflation, or changes in the value of the U.S. Dollar, can either raise or lower gas prices. If the dollar is worth more, prices usually drop, but if it is worth less, oil companies compensate for decreased income by raising prices.