What Can Be The Reason For Delay In The Periods?

I was supposed to start my period on the 3rd of April but I didn't so I took the a pregnany test the next day and it came out ngative. For a few days now I am having blood drops and it turns dry and today I had a pink drop. I took the EllaOne 5day contraceptive pill within 5 days after having sex..

What could be wrong with me Doctor please help?
P.S I have been under alot of stress because of this.