Advice needed

My little girl turned 6 weeks old yesterday and up until today she has not spit up one single time at all. She is formula fed and today she has spit up 3 times. 2 times early this morning and once tonight. When she does it I would not call it an excessive amount at all but the smell is concerning to me. It is clear looking with white in it. It smells sour though and I’m not sure if that’s ok? What should spit up smell like? It’s always like an hour or so after she eats.