Suspected infant reflux

My daughter is 1 month old and gets very fussy after feedings. She spits up a couple of times per day but most of the time she becomes very stiff and looks like she is going to spit up and makes a sour face. She is also very gassy and only pooping once every 3 to 4 days. She is exclusively breastfed and is gaining weight well. She was 8 lbs 10 oz at birth and was 10 lbs 6 oz at last appt (4 days ago) and just turned 1 month old.
She also does not sleep well unless held upright, especially during the day.
I suspect that I have a strong letdown and have tried to recline while nursing.
I have read on several sites that Nat Phos 6x can be very helpful with reflux. I have the tablets here but have been hesitant to try. What would the dosage be? Any guidance would be helpful so my sweet baby can be happy. Thank you!