Songs Get Stolen. (That's right, Zeppelin stole the into to Stairway to Heaven)
It's devastating when someone is out there making money off of your baby. It hurts. Just pray they're not more famous than you which, in the public's eyes, means the bigger group wrote the song. (If you were proactive enough to get a copyright, by all means go after them! You're lucky!) But often issues such as this might get overlooked, or not be properly covered, the copyright is only good on a full moon, etc.
Often, there is very little you can do. The internet is the wild west; once a song is out there, it's out of your control.
Don't despair. Don't freak out. Don't give up.
You should probably be flattered when someone blatantly steals your art. It means someone else believes your art is good enough to be worth stealing!
Remember, the greatest value you have as a musician is constantly delivering high value content. If you become known as a fountain of good ideas and songs, then you're power to defend your competitive advantage is iron clad. You're famous for songwriting not that one song.
People can steal your songs, but they can't steal your brain.
You should be constantly creating new and better songs to build up your reputation as someone who makes good songs. It's the same as with the software industry. New software can usually be reverse engineered and pirated within days of release. How do software company's stay profitable and keep competitors from swooping in copying their product at a cheaper price? It's the brand name that keeps you strong.
Apple makes well designed devices, not brilliant, super-high-tech gadgets using the most state-of-the art materials and software packages. Their customers think "Apple makes well designed devices that 'just work'". It doesn't matter than many other companies jump on the bandwagon to copy the iPhone. The iPhone is strong because of Apple's reputation for making well designed devices.
If David Byrne is in on a new project, I want to hear it! Yes, he wrote song great songs with the Talking Heads, but he's famous because David Byrne writes good music. You can't take that away.
Yes, you should get a copyright (PR & SR for my US readers). But it's not copyrights that protect you. It's your ability to constantly deliver high value content.