Uy, breakthrough bleeding yan, super normal talaga kapag nagsimula ka lang ng pills, lalo na first week pa lang, kaya wag mag-alala! Since nag-PT ka na at negative, and you started the pills on day 3 of your period, medyo protected ka na even if you had relations those first few days, pero the bleeding you're seeing now ay most likely just your body adjusting sa hormones ng pills, a few of my mum friends na nag-start din ng contraceptives experienced the same thing and it settled after a couple of weeks. That said, since you're breastfeeding, best to double-check with your OB na talaga na safe ang specific pills na yan para sa baby, because some progestin-only options are better for nursing moms, but the bleeding itself, almost certainly hindi pregnancy sign yan if you started protected. Keep monitoring lang, and if it gets heavy or lasts beyond 2-3 weeks, text your doctor na para peace of mind!