Could I be pregnant?

I really need someone’s help! I am currently 8 weeks postpartum, and me and my partner were having sex about 2/3 weeks after baby was born (yes I know it’s advised to wait until 6 weeks but my doctor just advised me its when I felt physically ready myself). With both pregnancy’s I suffered hyperemisis gravidarum (extreme morning sickness). So one of my first symptoms of pregnancy was feeling sick. I have been feeling sick constantly for the past 1/2 weeks. However every time I have done a pregnancy test it has come back negative. Could I be pregnant?

I got the contraceptive implant 3 weeks after birth. However, we had already had unprotected sex before then.