When Jaz was a bub, when I switched her from boob to formula, she started on lactogen & was constipated for the first few days. Then when they were sold out of lactogen & she had s26 she had trouble again, & again when we went back to lactogen.
I'd say its just her little belly getting used to it hun, never had the problem with Bri & Jesse's had the odd FF with no problems, so it doesn't happen to them all.