What Causes Missing Periods And Negative Pregnancy Tests?

Hello, I am 28 years old and I normally get my period every 25 days. There has never been any change in this pattern. I have recently started doing rigorous exercise and also take a diet shake (recommended by my gym). I had my last period on 14th April and now it is 18th May and I have missed my periods. I have done the pregnancy test and I am not pregnant. Pls help me with your response.