Umm??? I'm so confused!!!!

Ok so my son is 7 months old. I never breastfed him... So I was dry after a couple weeks. I've been dry this whole time.. I got the nexplanon in July and removed in October due to bleeding all the time!!! 6 months of bleeding was not working out for me! So I'm a week late on my period, (haven't had a period since I got it taken out) and I'm gaining weight, have pregnancy symptoms... So like 5 mins ago I realized that I am lactating... Like I'm positive its milk! 5 days ago I did a blood pregnancy test which came back negative.. I'm so confused and don't know why I would be lactating... Anyone know anything about this?