You still hold the copyright of the code. However, you've also provided a license for using/modifying/distributing the code. In fact, the very reason you can provide this license is because you hold the copyright of the code, which gives you the ability to license it under the terms you choose. You cannot at a later date, however, revoke the license for previous releases of the software without cause outlined in the license itself or some other formal agreement. Both GPL- and BSD-style licenses are based on this idea.
It's important to understand the relationship between copyright law and software licenses.