Aw nako, nakaka-alala yan! At 28 days old, mas safe talaga na i-check agad sa doctor para ma-rule out kung may underlying issue, kasi newborns dapat regular ang bowel movements nila, lalo na kung breastfeeding lang. One of my mum friends had the same worry with her newborn, and the pedia said it could be a latch issue or the baby's digestive system adjusting pa lang, pero definitely need proper assessment. In the meantime, gentle tummy massage (clockwise motion) and burping properly after feeds might help, pero please don't delay the doctor visit, better safe than sorry with newborns talaga, mama!