How do signs work?

So get this, I took a pregnancy test not too long ago and it came out negative. But I’ve noticed my boobs are different? I’m still young (18 as of November) and maybe puberty is still hitting? I have no idea what’s going on but my breasts feel and look fuller and are no longer a tear drop shape but more circle. My nipples and breasts aren’t sore at all either but last week it felt like growing pains. What’s going on? And should I be worried (also I do have birth control- a IUD)