IUD Removed. Unprotected Sex

So, my IUD was removed on 12|5|19. I started bleeding the next day and it was kind of heavy, but lasted like a normal period. They say fertility returns immediately, so I actually counted it as my first period. Me and my partner had sex on 12|18 and when I’m looking on my chart, it’s the day that I had the highest chances of getting prego. We did not use a condom and now, I am feeling like all sorts of crap a week later. The nausea and the HORRIBLE back pain is just beyond me right now. I felt this exact same way when I was prego with my son. My body IMMEDIATELY started to warn me that I was pregnant. 😖 What do you guys think?