YEP!! Sure is. By uploading a song and making it available for others to access for free is distributing copyrighted media illegally. It is the same as if I bought a CD in a store and went home and made 10 copies and gave them to my friends. It's illegal to distribute copyrighted music, software, etc., even if it's for free.
Just FYI - as LDB stated earlier, it's not the copying of the song that's an issue; it's what you do with that copy. Sounds like you are very consciencious about this and you're doing the right thing by trying to remove your upload.