At this time of year, we hear from students who say, "Grrr! Why does Turnitin show a plagiarism percentage even though I have properly quoted and cited all my sources? I am not a plagiarizer!" The answer is that Turnitin produces a similarity index NOT a plagiarism score. Turnitin will highlight ANY matching material in a paper—even if it is properly quoted and cited. Just because it appears as unoriginal does not mean it is plagiarized; it just means that the material matches something in our databases.
Perhaps your assignment specifies that all your source materials must be summarized and/or paraphrased rather than quoted. Well paraphrased and/or summarized material may not show up as unoriginal in Turnitin - but must be cited in any case. Many students need help learning to paraphrase, summarize, and attribute their source materials; ask your instructor or writing center for help if you don't know how to do this. You can also get some helpful tips from Purdue's Online Writing Lab (OWL) and from WriteCheck's resources section.