Hi, My Last Period Started On 4nov. On 13 November

Hi, my last period started on 4nov. On 13 November I had an ipill after unprotected sex with my husband. I normally have regular 26/28 day menstruation cycle, so my periods were due on 2dec. There was minor spotting post the ipill. I took 2 home pregnancy test on 12 dec and 19dec. Both were negative. Still not got my periods. I am 38 yrs old having a 10 yr old son. Local doc has recommended tab duolution l for 5 days. How will this tablet help? Why r my periods so late