First off, there is a difference between entrapment and merely setting a trap.  Laypeople often think these are just two names for the same thing and that all police stings are thus illegal.  It's easy to find people who've learned that this isn't true - just go to your local jail.

There is nothing illegal about posting bail for someone, even if you know she's guilty as charged.  You can't get into trouble for that.  

It is possible that the police are setting you up so they can record you admitting to past exchanges of money for sex, but your unsworn, out-of-court statement will not even be admissible unless there is some corroborating evidence (this is called the corpus delicti rule).  While the lady's testimony against you would normally be additional evidence, the corpus delicti rule requires the corroboration to come from someone who was not the accused's accomplice.  I'm not certain of this, but I believe each participant in an act of prostitution is an accomplice of the other.  If I'm right, then the young woman's testimony would also not be admissible unless there was additional evidence.

The police may hope you and the provider will have sex for money again while they are watching, but a sting designed merely to get men to talk about prior acts seems unlikely to lead to any convictions.  Having such a conversation is not a crime, so there doesn't seem to be much risk involved in your situation.