I had sex 8 days after my period ended and 13 days since the first day of my period. He didn't pull out its been 16 days and I'm supposed to get my period today but still no sign of it. I spend all day yesterday throwing up' don't know if its related.. we are both scared
When should I buy a pregnancy test ?

Pointless as they only detect increase in pregnancy hormone around the time of your next expected cycle..

Most likely another two weeks to test. You can try one in a week but if negative, I would still retest a week latter.

* ** pulling out is not a real method, it is very ineffective. If you have sex, get on birth control and use a condom also

Taking a test is safe, since it simply measures the presence of hormones in your urine. However, taking the test at this time is not likely to be effective. The accuracy of these tests increase greatly from the time you miss your period. So the accuracy on the day you expect your period is going to be under 20%. It will increase to over 95% about 2 weeks after you miss your period.