How Can My Pregnancy Test Say I Am About 3 Weeks Pregnant When I Only Had Sex A Few Days Back?

Hi Dr Please can you help me understand I took a clear blue home test that tells how many weeks you are, I have been on holiday since 21/8/22 to 30/8/22 I had unprotected sex on holiday , I have been taking period delay Norethisterone tablets from 20th /8/22 as I was due on my period around the 26th /8/22 I have got home and done a pregnancy test that states I’m 2/3 weeks ? How can that be when I didn’t hVe sex 2/3 3/4 weeks ago ? I only had sex whilst on holiday ? It’s now 3/9/22 how can it say I’m 2/3 weeks ?