*This content was adapted from "Copyright Infringement" by University of Minnesota Libraries' Copyright Program is licensed under CC BY-NC 3.0
If you discover your own work being used or shared unexpectedly, you may first wish to consider the possibility that someone else is making fair use of your work. However, copyright infringement (sometimes unintentional, sometimes malicious) is not uncommon. If your work is being infringed, there are a number of steps you can take.
Don't wait too long! There is a statute of limitations on copyright claims - if you wait more than three years after you find out about the potentially-infringing use, you may not be able to take legal action. (In some jurisdictions, the statute of limitations is understood to begin at the time of infringement, not the time you find out about it! Cases in these jurisdictions expire even faster!)
However, don't rush in too quickly! It is easy to make very sweeping arguments and threats when you feel your rights have been violated, but this rarely plays out well in the long term. You may find that some infringing uses are not worth your time to pursue. Note that you cannot lose your copyright ownership by failing to police your works. People are sometimes confused about this, because you can lose trademarks through neglect, but you continue to own your copyright for the full term, or until you transfer it away.