What Causes Breast Tenderness And Delayed Periods While Having Negative UPT?

I am 22 years old and have irregular periods with cycles between 28-31 days. My boyfriend and I were intimate the 13th and 14th of June. My last period was on May 18th and today it is June 24th. I am not more stressed than normal nor am I overweight or underweight. I have recently began to exercise more than I had before, but there have been times where I have increased the amount of times I exercised before and still got a period. I took a pregnancy test on the 22nd and the 23rd and both came back negative. I have breast tenderness, fatigue, very mild cramping and noticed that I have been more constipated than usual; why haven't I started my period yet?