2Positive pregnancy test at 3 weeks

Marilyn
One done at home one done at Doctor. In some cases I heard it's possible to test that early. I'm thinking I am more like 4 weeks based on all symptoms. My period is do today. My last period was Started 10.8 and finished 10/14. Could I have gotten preg. The 16 or 17th? Or my fertile days of the 21-27? Anyone know how far along I could be? Online is confusing me.