Could I be pregnant or ..?

I'm 19 years old and had unprotected sex for the first time exactly 7 days ago. Right away I started thinking, "oh no, what if I get pregnant" then I told myself that it was probably rare that someone could get pregnant their first time not using protection; but I'm not sure. When can I take a pregnancy test? Also, today my breast were really lumpy, like disgustingly lumpy. It kinda creeped me out because they were never like that before. Is that a sign that I could be? like could it be milk forming? I'm so clueless. How soon can you expect symptoms and what not Someone give me the low down please! :)