Hello I Have Something To Ask...i Had Sex Last December

Hello i have something to ask...i had sex last december 14 with my boyfriend. It happened every night until 16 of december but its all done with widrawal method. I had my period decmber 28 and it only last 2 days and not regular period its just like ..no so much blood came out.
We had sex again december 31 and january 1 2016 and still widrawal method. And until now my period is not yet coming.. I took 3 times pregnancy test already and its negative..what should i do? Please need a good advise to what happend to me..thanks..