I had sex a week before my period and now my period is late coming. There are actually a couple of possibilities here. First, it could be related to your age.
Young women tend to have irregular periods.
The whole system seems to work itself out and get into a routine as you get older. Even then, a woman's period can vary 8 to 9 days. It could also be related to your activity level. If you change your usual exercise routine it can affect your cycle. Things like travel, diet, stress, illness and medication can also affect your period.