Playing poker is the same as living a life: there are also black and white stripes. Poker players made up special terms for such stripes – downswing and upswing (or downstrike and upstrike).
As you can guess downswing is series of misfortunes, universal injustice, bad luck and other epic words players use to describe their bad games. Downswing can be distinguished from poor playing with such symptoms. You've chosen the proven strategy, but regardless of all mathematic expectations you have bad cards. You lose with the strong hand to the opponent with the weak one. Downswing is a real and natural phenomenon that can be observed in the play history of every player. Such famous poker players as Gus Hansen and Phil Galfond had long periods of downswing. Although poker is considered the game of skill with no place for fortune interference, but we all sometimes have to face the back of this lady. You can't determine how long this period will take place. Length of downswing period is individual for everyone. Losses during the night or regular losses can be the example of the downswing.
Downswing can be dangerous because players lose faith in their forces, their skills and what is worth their faith in the game. Downswing has some more negative consequences: from losing big sums of money to passive/aggressive condition named tilt when players feel frustration or confusion because of game failures. The player can't control himself; he changes his game strategy and consequently loses all his money (bankroll).
If you feel that you are in a period of downswing keep to the general rules: don't change your game strategy, choose lower bet limits, play on fewer tables, or stay away from the game for a little while.
There are always good forces versus evil. The same is in the poker, long-playing players have the most pleasant white periods of pure luck – upswing. You are dealt only straights and flushes, your cards grow the sum of your money increases, little chances give you great winnings, you opponents give up – total positive and happiness. Unfortunately, upswing can also be dangerous. The player gets used to the perfect game and when "luck" is over, he has tilt condition.
Remember that downswing and upswing will be over sooner or later, you shouldn't get the label of underdog or lucky dog, as black/white stripes would be replaced by alternating winnings and failures.