Well, you had sex on day 18 of your cycle . . . late for conception typically. Periods can be late all the time and being sick, in pain, on meds--- this can cause that. Stress, changes in routine, weight gain or loss too. I'm guessing you are just late. If you don't get your period in a full week, then go ahead and take a home P test. But again, I am guessing you're just late. good luck