had sex on my last day of my period my bf came in me

While not impossible it is unlikely you are pregnant if you had sex while on your period.  How long does your period usually run and when do you ovulate?  Sperm only remain viable for about 5 days so if you had sex on say day 5 of your period and you ovulate on day 12, then it would be unlikely.  

The earliest you can test reliably is when you are due for your next period.  Use your first morning urine, it is more concentrated and more likely to show a positive early.  Follow all directions, especially times, closely.  Remember, it is either yes or no, not almost or maybe.  

False negatives are common testing early.  False positives are rare.