Delayed Menstruation but negative PT

Good am mommies. 9 month Postpartum po. worried lng po ako. June ako nanganak. ngkaroon ako ng menstruation last Dec. 2020 pero spots lng sya. then January 21 ulit ngkaron ko and it lasted for 2weeks. Then Feb until now di pa ako ngkaroon uli. I tested Negative sa Pt twice na March 24 yung first then today lng yung ika 2nd but still negative nman... nagtetake ako ng pills since Dec. Breastfeed ako. Minsan na din kmi ng s*x ng mister ko.. i expect na Preggy but negative nmn sya.. Is there any reason kung bakit na dedelay ang menstruation since negative nman po sa pt..