6 weeks post postrum pregnant??

I'm 6 weeks post postrum & I haven't got my period. So today at my 6 week checkup they decided to do a pregnancy test & they told me it came out faint but you could hardly tell.. I then came home & took myself a test and it came back negative is it because I have left over hcg from having my baby or can I possibly be pregnant.

Background story.. me and my SO had protected sex at 3 weeks post postrum & once he finished in the condom he took it off and we went and took a shower. After we got out the shower playing around he put it back in me without a condom & thruster only about 3 times & then took it out but didn't finish in me. Is it possible for his sperm to have been alive since we had sex 1 hour before he did this..