You don't lose the rights to your code. The original copyright owner still owns the copyright on their code. That's why you have to follow the rules of the license they gave you. You can still distribute your code under whatever license you want.
> You can still distribute your code under whatever license you want.
That actually isn't true.
> You must license the entire work, as a whole, under this License to anyone who comes into possession of a copy.
You are required to distribute it under GPL v3. I don't know enough legalese to determine if distributing the modifications alone as a parallel license is allowed.
This is only true if "your" code is a derivative of someone else's code. Putting code that's actually original into a project that also has GPL code in it doesn't mean you have to GPL your code.
Distributing binaries with mixed sources is where it gets hairy.
