DD is the same age. She poops also about every 5-7 days, thats just her. We have mege issues with reflux, but here they say most babys do, their tummys havent finished developing and wont give antacids. She has only just stopped having colic every day but some days she is a bugger to feed, twisting and turning, drinking 20ml every 5 or 10 mins then goes to sleep (these days I susspect she has alot of wind) and I will give her warm water to try work out the snacking, make her hungry and have a proper feed and it mostly works.
What other formulas have you tried? perhaps its that formula? it can be tricky to find the "right" 1. I know another lady here has baby same age and had a hell of a time getting the right formula for her baby.
Without knowing the whole thing, I think your right to question your ped