help with my HIV dilemma

Hi,
Approximately 20 hours ago I had unprotected oral sex & unprotected sexual intercourse with a brothel worker for half an hour. I would speculate and say she is in her late 30s. I asked her how long she was doing this job for and she said for the past 2 months. She told me that she gets tested once a week but HIV doesn't show up until at least 6 weeks, 3 months, 6 months etc.

Now I know it was a stupid decision but how likely do you think it would be that I have contracted HIV from this single encounter with her? I'm not too sure if she has HIV or not, especially considering she is a brothel worker (brothel workers have low levels of STIs in Australia, and also by law, she should have informed me that she had some sort of STI if she had one but she didn't so im assuming she didnt?) but her willingness to have unprotected sex makes me wonder how many other guys she has had unprotected sex with too.

How effective are those PEP tablets? Would it be necessary to use them, or do you think I am worrying for no reason?

Thank you very much.