Hi mama! Honestly, 15 days is quite unusual especially kung regular ka talaga, at the cetirizine could be a factor kaya I'd really suggest checking with your OB ASAP, better to rule out anything, lalo na at 7 months postpartum pa lang ikaw at nag-CS ka. A few of us had delayed periods from meds, stress, or just hormones settling after birth, pero ang negative PT is reassuring naman, pero your doctor can check what's really going on talaga, whether it's the allergy meds, breastfeeding, or something else. Wag mag-alala too much in the meantime, pero don't wait too long to get it checked, okay?