Projectile Spit Up??

This is my second baby and she is now almost 4 weeks old. On last Sunday, she projectile spat up for the first time. Last night, she did it twice more. My doctor said she's probably just having air bubbles, but I try to burp her and nothing comes out. Is there any way I can prevent more projectile spit up? Should I be worried about it? My first never projectile spat up.