I would ditch the formula... its likely either causing her problem or making it worse.
How is your letdown? If you have a forceful letdown (sprays out like crazy) she may be gulping too much air and it will cause her to be gassy. If this is the case, let her latch until your milk lets down, then unlatch her and let it spray out into a towel until it slows or stops. When it does, relatch her and let her nurse ONE BREAST ONLY each feeding, then use the other breast for the next feeding. A forceful letdown will slow quickly as long as there is no stimulation, ie let it spray itself into a towel instead of having her nurse, will tell your body to ease up on the letdown.
Breastfed babies can go up to 2wks without pooping because your milk is perfect for her body, makes what she needs ,and her body uses almost everything in the milk because its perfect. Giving her formula, even a little bit, messes with this perfect food combination and confuses her body, especially this early on. So breastfeed only, you ahve plenty of milk for her and there is no reason to supplement. And relax, it will pass (no pun intended) and she will be fine.