'm Experiencing A Missed Period. I'm 23 Years Old And

'm experiencing a missed period. I'm 23 years old and I've recently I've quit my running career as a division 1 enduranced trained athlete. I've been running for 10 years and I've now decreased my intensity level due to no longer competing. Back on February 14th was my last period that I had but I've now missed March and I'm currently a month late on my period. I have no signs and symptoms of feeling sick or anything along the lines of being pregnant and I'm currently on birth control. I've also taken a home pregnancy test twice showing that I was NOT pregnant. I was thinking that it was maybe due to my recent change in physical activity that maybe my body is trying to adjust but from what I've heard is that women miss their periods with increased exercise but mine seems to be decreased exercise. I'm not sure if that's maybe possible, or maybe stress, or if I really am just pregnant.