Postpartum Period Late

I had my first postpartum period in June. (My son was born last July, but my period was delayed due to nursing.) I’m not sure how long my cycle is at this point, but if I track by the average 28 day cycle, my period is 7 days late. I took a pregnancy test on Saturday, and it was negative. Does anyone have experience with postpartum period while nursing (we are currently weaning)? Is this normal? Could I still be pregnant even though the test was negative?