heres the trick- hanging out with a new group of women, pregnant women, etc can change your body clock. that means delayed periods, periods getting skipped, or early periods.  additionally, most women freak out after having unprotected sex, condoms breaking, etc. so even if you aren't pregnant, and youre freaking out about it, your stress level will almost guaranteed delay your period at the least, if not force your body to skip it.

there have also been several documented cases of women psychologicially convincing themselves that they are pregnant, and their bodies changing respectively.

best thing to do?  RELAX. if you are, congratulations. if you aren't, c'est la vie. continue taking home tests (not until 19 days after you last had unprotected sex or exposure, etc) and if you are still unsure about it, go to your gyno.  DO NOT, however, completely rely on the blood tests. there have been several cases of hcG not showing up in the blood until several months into pregnancy.

tl;dr- dont smoke, drink, or do anything stupid. do keep up with vitamins. act normal, and relax. you will find out with time.