Aiyoh mum, this is quite unusual lah, 3 months apart or longer periods can happen after delivery but at 9 months postpartum it's worth getting checked out, especially since you're fully formula feeding (so not the breastfeeding suppression effect). A few of us mums in my circle had irregular cycles after giving birth and most turned out okay, but better safe than sorry, next time you have a free moment, do pop by the clinic to see what's happening, because it could be anything from hormones still settling down to something needing attention lah. Don't worry yourself too much, but definitely don't skip the appointment like last time okay, even if your period shows up two days before!