sex 3 days before period and no period,pregnant?

i had unprotected sex with my boyfriend about 3 days before my period was supposed to start. my period is very regular. and im not on any birth control. my period was supposed to start on Jan 2 or 3. and here it is Jan 21 and i Still havnt started my period. ive read that its highly unlikely that i could be pregnant but lately ive had extremely sore breats (more so then before my period, even a flick could hurt enough to make me tear), i pee nearly every 20 mins(not exactly but it seems that frequent), and ive been getting sooo tired. i normally dont sleep until about 2 or 3am and ive been going to sleep at about 9pm or 10 pm. i havnt really thought about being pregnant because i knew that the likelyhood of getting pregnant just before the start of your period is very low but im nearly 3 weeks late! i talked to a friend and she suggested that i take a home pregnancy test asap. but i have school all day long for the next four days and wont have time to go out before the stores close. and i dont know if its too early to detect it. im a poor college student right now so i cant afford to spend a lot of money on a pregnancy test if it will be too early to tell. so now im here seeking your honest opinions. :-D thanks
-sam