Contraceptive failure. Sex that day.

Since about a month, I had been having unexplained fatigue, nausea throughout the day for about a week and in the past week have gained 1.5 kgs tho I've been exercising. My diet has been in control mostly as my SGPT levels were 133 and I was advised to eat healthy. I took 1 home pregnancy test and a BhCG test 2 weeks ago which came negative. Since then I finished my 4 day periods one week ago. I have also been on a combined oral contraceptive pill for 6 months and haven't missed any pills tho I did t ake Metronidazole as treatment for amoebiasis in the last week of April. I had diarrhea in that week tho I never missed my ocp or vomited after taking it. Could I be pregnant because the fatigue still persists and my periods this month were slightly lighter than usual. Please advise Note: I had taken an ECP as an added precaution last month on my supposed day of ovulation, as I had unprotected sex that day.