Baby keeps spitting up

My baby is 3 weeks old and is formula fed (Similac pro advance) with Avent natural bottles. After every feeding I burp her and sit her up right for at least 20 minutes but she spits up and milk also comes out of her nose. She is still gaining weight. Could it be her formula? Or her bottle? We have our 1 month appointment on the 22nd but I figured I’d ask here first.