I Had Unprotected Sex On 4th Of December 2015 Used

I had unprotected sex on 4th of December 2015 used withdrawal method. I took pregnancy test after few weeks it was negative. I got my periods but late. Then even in January 2016 periods were fine. Since February I am getting lighter periods. We did not do sex after new year. And also in April I got much lighter period lasting 2 days and flow was quite low. I worried as I am unmarried. Please help