If my contract states I own the copyright to my artwork, if the client hires another artist to modify my work with out my knowledge or permission is that a violation of my contract?
Modifying copyrighted work without the copyright owner's permission is an infringement of the copyright. Only the copyright owner can legally make or authorize "derivative works." Your contract may have authorized certain uses of your work, so before you take any action you should consult with a qualified copyright attorney to review your particular situation.