I have a 26 day old newborn, she has been on formula for 6 days (nan ha), she has not pooped in over 24 hrs and struggles and is in pain when she passes gas. should i be concerned that she could be constipated?

Not necessarily: If they poop rabbit pellets or hard stool it is constipation. Infants have poor abdominal muscles & poop laying flat, so they need their breathing muscle to assist and often fuss a little with the process. I don't assume this is pain more than work. The belly will get hard as they work it out. If it is hard an ounce of full strength prune juice 1-3X/d can get her moving in a few days.