Why is my period blood brown?

i am 13 been on my period for about a year maybe two.. in the first part of my period me and my friend had our preiod the same day. sooner or later that changed. usally when im late for a period i get nervous. of course i never had sex with a guy, but i still fel like maybe i wore some of my boyfriends shorts or something. so i freak out untill my peiod starts. well right now i was about 5 or 6 days late.. i got kinda nervous and freaking out and i was always thinking about it. before i started, which was today, i started doing some abb workout and started eating differently. i read some sights and some said that, it would change the date of my period. well apperently it did. but my question is, why is my blood brown? i can pretty much tell im on my period. it flows kinda light and has red in it. its kinda like spotting. ive read on a website that usually brown spotting is due to pregnancy. so as usual i started freaking out. please help me im constantly thinking about what it is. what makes it even worse is im having bad cramps. i dont want to buy a pregnancy test because my dad would kill me for going down the row. im only thirteen and im confused..so please help.