gwaan83, there is little I can add to what has already been provided you by justalayman. Sloop John D's reiteration of justalayman's advice to register your music now is an indication of how important this step can be to you in asserting your rights.
There could be problems with your registration, however, and this is why you should seek out the help of an attorney in your area. Having an attorney assist you now can make a difference in whether you will be able to recapture the rights in your music currently being claimed by the unauthorized users, and whether you can then be compensated for these unauthorized uses.
Sloop John D is correct that you can register your music now IF it has not already been registered by those currently claiming rights to it. If they have already registered your music under their names, they will be the presumed copyright owners of the music, and it can take a lot of time and effort and evidence (and oftentimes a lot of money) to overcome this presumption.
If you created the music in 2011 and you published the track online or elsewhere at or around that time, but you did not register the music at or around that time, then you will not be eligible to collect statutory damages for the infringement of your works. This would be the case even if you are able to register the music now. If you can register the music now, however, then you could potentially collect on any provable profits made by those using your work, or on any provable losses you have suffered as a result of the unauthorized uses.
You will need to register the copyright in your music if you want to file a copyright infringement suit against your infringers.
When you locate an attorney to handle this for you, the attorney may recommend, as a first step, sending a cease and desist letter to the infringers, containing a demand for compensation and a threat to sue if the demand is not met. A C&D letter sent by an attorney can often intimidate those in receipt of such a letter into settling the matter, to avoid a costly court action.
I suggest you follow the steps outlined by justalayman in getting your music removed from its locations online, and also access through the links he has provided the information on copyrights and infringement.
And, again, contact an attorney in your area - soon.
Good luck.