Hello, I am Dr. S and look forward to working with you towards an answer to your question.
The arching of the back during certain feedings along with some improvement with holding her upright is commonly seen with GER (Gastroesophogeal reflux). This is common when switching between breast feeding to formula as the amount with breast feeding varies with each feed and is the speed of the feed is partially controlled by the infant and their suckling. With a bottle, we tend to feed more with each feed as we are going off a set amount. Things that can help if it is GER is exactly as you have been doing, feeding in a more upright position and keeping head elevated for 10-15 minutes after each feed helps gravity keep the food in the stomach and less likely to move back up. Frequent burping also helps to remove extra gas in the stomach which also helps decrease distention and symptoms. Doing smaller, more frequent meals also allows the food to move out of the stomach and again decrease distention. Formula can be more constipating for some infants, which may be why you are seeing more straining with BM's. The body is usually pretty good about regulating itself and should improve after a few days.
Occasionally, if this does not work, some pediatricians will recommend thickening the formula, however it would be good to wait until you can contact your pediatrician on Tuesday as every pediatrician has different ideas on this. Some will also use medication such as ranitidine if the symptoms concern.
So smaller, more frequent feeds, burp frequently, keep head elevated with feeds and for some time after.
As long as your infant is otherwise acting normally, making normal wet diapers, BM's ok, acting ok, no trouble breathing, the most common cause of similar symptoms is GER.
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