BFP why is it coming up so strong.

My lmp was on the 14th August 2018. My next period is due on the Monday the 10th of September 2018. I test yesterday and it came up extremely strong with very dark lines! And I bought a clear blue and it's confirmed 2-3weeks I am so extremely blessed and happy but how is this coming up so strong since my period was only 3 weeks ago.