12 days late

Can anyone help please, I was on the shot for about 8 months and gain some weight, so decided to switch to copper iud, no hormones so maybe no weight gain lol but I knew I was definitely going to have my period. I switch to copper iud in July had some spotting after the insertion but was fine in Aug 3rd I got my first period and now I'm late 12 days I took a pregnancy test yesterday but it was negative. Any one knows anything about this or going through something similar?..