Oo, super normal yan sis! Kahit may withdrawal pa, ang breastfeeding talaga ang main culprit, nagkakaroon ng hormonal changes na nakakapagsagot ng irregular periods, lalo na if exclusive breastfeeding ka pa. Isa sa mum friends ko, parang 8-9 months bago nag-regular ulit after the first period came back, kaya don't panic na.
Since negative ang PT mo at sure ka sa protection, I'd say just monitor lang for now, pero honestly if nakakabgit na sa iyo or if mag-extend pa ng matagal without period, mas better consult your OB para masigurado at peace of mind mo, they can rule out anything else at guide ka properly since you're still breastfeeding.