What Causes Delay In Periods?

Hi

I have currently started becoming sexually active. I haven't yet engaged in intercourse but me and my boyfriend have been as they say 'fooling around'. We've been being careful of course. I'm not yet on the pill as I have been waiting for my period to start to begin taking the pill. However my period was due on the 24th March. It hasn't come yet therefore I'm 3 days late, but I took 2 pregnancy tests that day and they both came out as negative.

I have been researching that missed periods can be caused by stress, and the week leading up to my supposed period date I was quite stressed out that I may be pregnant...therefore stress could be the cause of the late period.

Also, I've researched others symptoms of pregnancy and I have none of them.

Are the any suggestions as to why my period my be late