Spitting up

Sarah
My 3 weeks old has been spitting up since day one. Everytime he feeds he spits up. In between feeding he spits up. Everytime I lay him flat he spits up. 
Knowing that he is exclusively breast fed. He sleeps in an inclined rocker (if anywhere else he would spit up) & I make sure he's burped after every feeding (even though sometimes it takes more than 20 minutes for him to burp. 
It's so frustrating. I already asked his pediatrician she said that it's not considered a problem till he is about a month old. Then we can address if he is still spitting up. 
Any thoughts ? Any one went thru this ? 
Thanks.