Ok need help or at least to know I'm not alone

So I have always had regular periods until last month I spotted instead and now I'm late. I am sexually active but we always use condoms and took a hpt and got negative.I have been super stressed with my summer job and job stuff for the upcoming school year. I have the knots in my back like I'm gonna start. What is going on with me? Am I the only one this has happened to?