What Causes Delayed Periods When Having Unprotected Sex?

Hello...I am 28yr old and married for 2yrs.We had a protected sex until last year December.12days after my periods in December i.e on 16 we had unprotected sex and later in that month 3times it continued.I missed my periods in January and when I did pregnancy test it gave me negative but I had lower abdominal pain.In February my periods were very late.Whenever I had unprotected sex,I missed my periods the next month and if I had a protected one my periods are normal for 28-30days.After August 31st my periods are over and now when I had unprotected sex 4-5 times in September, I missed my periods on September 28 and October 28.I did pregnancy test on 21st September but it gave me negative result but I have mild lower abdominal pain.I did not have any other symptoms of pregnancy.Please advice.