Before you buy a pregnancy test read the guidelines on the back because they have you wait a certain amount of time before you can take it for full accuracy.
Also, it may just be that your body's just weird. I was regular for a LONG time, and once, this past semester, after I had (unprotected) sex with my boyfriend, I had similar side effects and thought I was preggers. But, for some reason, my body was just freaking out similar to how yours is (except minus the itchy nipples, that's kind of strange). Definitely not pregnant though, my last three periods have proven that. I know it's a scary and tense situation, but the worst thing you can do is freak out hardcore, it makes things worse. Try to keep as cool a head as possible in the meantime, you know, before you find out anything definitive.
Good luck