Ay normal talaga yan, especially pag nagbreastfeed ka pa! A few of us moms experienced the same, yung period namin was super erratic for months kahit regular kami before, and it's because breastfeeding can delay or suppress ovulation pa lang. Since nag-PT ka na and negative, you're probably fine, but if you're worried or may signs of pregnancy pa rin, better check with your OB just to be sure at para sa peace of mind mo na talaga.