Put very simply, human trafficking is the exploitation of someone else for your personal gain, so not all consensual sex work has anything to do with human trafficking. In fact, they're completely different.
Where we look at human trafficking is when there is another individual who is threatening, coercing and enforcing someone into the commercial sex industry and where they are profiting. It's also where you see individuals not feeling able to exit, again, because of the fear and the threats imposed on them.