Breastfeeding related or pregnant again

So I finished bleeding around 2.5-3  after my son was born and he's now 2 months on Thursday and I haven't gotten a period again yet. I am still exclusively breastfeeding, but with my daughter I was as well and started my period right back up bled for 2.5 weeks then at 6 weeks I got it again, and was very consistent until I had my iud placed and then I never had a period, but this time around I could of gotten pregnant I've taken a few tests all been negative. I had sex the first time at 5 weeks 1 day post partum. Should I be concerned about pregnancy or just assume it's cuz I'm bf that I have no period?!