Please help!

Ok so something strange has been happening lately... As of today I'm 15 dpo and I'm three days late on my period whereas I'm never late. I've had a stomach ache since when my period was meant to begin and from. The evening of the first missed day I've had cramps on and off. I did a pregnancy test on the second day and it came back negative but my period shows no signs of showing up. I'm really worried as because of the stomach ache I don't want this to be an ectopic pregnancy or something like that. I was hoping some of you knew what this is or had this happen to you. Really need your help. Thank you