You don't qualify for a PEP. It is generally recommended for someone who had an exposure with a known positive.
Your risk is miniscule, it's very unlikely that you contracted HIV from this exposure. Female to Male HIV transmission is relatively difficult and it is often seen that it takes multiple exposure with a known positive for transmission. Whereas, you describe to be unprotected for only 20 seconds. Also, you are only guessing that this CSW was positive. Most CSWs are very careful about their own health, they test at correct intervals and insist on protection for their own safety.
You need to take it easy, get tested at 4 weeks with a IV Gen HIV test. You should be okay.
PEP is a matter that needs to be discussed with your ID doctor and not on an online forum. Though, if you ask me, I don't see a point as explained before. Good Luck.