I designed a bumper sticker including one sentence. That sentence was from a popular song, however i changed one word in the sentence.

Generally speaking, slogans that do not serve to identify the origin or sponsorship of goods cannot qualify as a trademark. Moreover, slogans are not copyrightable. You should consult with an attorney regarding the precise facts of your situation, including how the stickers are being distributed and what the goods and services are to which they relate (which you do not provide in your inquiry).