questions on early symptoms

Bree • Mommy to a 5 year old boy & due with boy number 2 in December 💙💙
Hello! My name is Bree,
 I have a little boy who's 4 years old. Although it's been a while I still remember most of my early pregnancy and symptoms with him. When I found out I was pregnant I remember being surprised, and in hindsight the only symptom I had was fatigue. I didn't really start feeling other symptoms for a few more weeks.  My period was not due until 4/30...but on 4/19 I came home from work and for the 3rd day I was extremely tired, sore breasts, and heartburn. So around 6pm I took a pregnancy test and to my surprise it was positive, faint. But definitely positive. I took 4 more the next morning, 3 different brands. All positive. I called my dr, she got me in on 4/21 told me yes it's positive, faint but only because it's so early, she said I was around 3 weeks and 5 days (which puts me at 4 weeks today)  My question to you ladies is, what in the world made my test show up positive so early, and why are my symptoms so strong already? My doctor doesn't seem concerned at all, told me everything will be fine and she'll see me back in a month for my first official OB appointment. Has anyone experienced anything similar??  Thanks in advance!