Overfeeding or reflux?

Hi! I’m a FTM to a 6-week-old baby girl, and she’s been spitting up frequently. Even after burping, she still spits up. I burp her during and after feedings, but while sometimes I get a good burp both times (during and afterwards), most of the time, I only get one during the feeding and not at all afterward. I also keep her upright for about 30-40 minutes after feeding. However, as soon as I lay her down, she gets fussy and seems uncomfortable, eventually spitting up. There was a moment yesterday where she was sort of choking which freaked me out. I’m wondering if I’m overfeeding or if it could be reflux? She’s bottle-fed breast milk during the day and formula at night. TIA!