Is what i'm doing plagiarism??

I was looking for a place to start when composing a song on the piano. I found a video on youtube with a girl playing a song from a movie. She made up the chords herself, but she didn't make up the song. So I was bored of the repetition in the song, so I changed it up and added a lot of notes and stuff. I tell people that I didn't make up the main melody, but I did make up the song. This is true, right? Am I plagiarizing?