The problem of measuring heart rate while cycling

I was cycling yesterday, in the first 11 minutes my heart rate did not change even though I was riding vigorously. And then after 11 minutes of driving the heart rate jumps from 91 to 146 beats at once as seen in the picture below. It was as if he registered no changes in heart rate at all for the first 11 minutes. Does anyone know what it is and why this is happening, otherwise it is not the first time this has happened.