Period 2 months late neg tests 5 month postpartum?

I had a baby in feb this year I got a plasma transfusion at the hosp a few hrs after delivery and I stop bleding 3 weeks after the birth. Then a month later I had a light 3 day period in april then a month later may I had a 5 day period and I havnt had one since may 14th. I have taken 2 test diff brands and both were neg. Ive exclusively feed since the birth so its not like my period just hasn't returned. Could it have stopped again cuz the tranfusion? With my first after the birth I had no transfusion and bleed 8 months? What is going on? Thanks. Oh and I'm not on hormonal birth conrtol.