Aiyoh, don't panic lah, missing a period on IUCD is actually quite common, a few of us mommies with coils experienced this too! Since your UPT came back negative after 4 days, I'd say it's safest to go see your doctor or gynae just to confirm everything is in place and there's nothing else going on, especially since you're a first-time mum and probably feeling extra worried. Could be hormonal changes from the coil, stress, or just your body adjusting (mine took quite a while to settle into a rhythm), but better to get it checked out properly so you can stop stressing, ya?