We had unprotected sex and he finished, my period is late now, but preganews came negative

I did unprotected sex on 17th of Oct.n when I asked my bf whether he ejaculated or not,he said he did. But the sex lasted only for 20 seconds. It was our first time.after 2-3 days I used preganews and the results came negative. And my date of period was 29th of month.I missed my periods. Please help me what should I do now ?