What Is Causing Delay In My Period?

I am a week late on my period. I was suppose to start on the 19th of july. I usually start on time but something is different this time. I am getting every symptom possible for pregnancy including booger like discharge (only once) which was today the 27th. I have taken 3 pregnancy tests and all come out negative. I had an ectopic pregnancy in march but i don t think it has anything to do with this. After the ectopic pregnancy my periods have been on time. What can it be?