Still irregular with the IUD

I've had Skyla for about 4 months now. My periods have never been regular until I started the pill, but since I turned 18 I decided the IUD would be better insurance wise (full coverage while pills $20 a pack). The first three periods were on track but got lighter and lighter so I thought I was going to maybe stop with my period like some people do. This month I was a week late and my period came full force like nothing happened and is lasting more than my usual 3 days. My boyfriend was scared and wanted me to take a pregnancy test even though we use condoms and pull out along with my IUD (college kids don't need a baby lol). It came back negative of course, but this is super weird. I hope my period doesn't start being super irregular again. I had a period with normal flow for four months before after 6 months of no periods at all and I wasn't sexually active then!