Spitting up with nursing but not bottle

I pump and nurse my son, and give him bottles when we are out and about or when I’m tired of nursing. I’m starting to notice that when he just nurses on demand he has been starting to spit up. When he has more bottles he doesn’t at all. Has anyone else experienced this? What do you think the reason for that is? He is one month old.

Edit: I realize its unclear, but he does not spit up while he is nursing, just when he has been nursing a lot that day. He spits up at totally random times.