Breast fed babies many times don't poop as often, but are also less likely to get constipated. Formula fed babies poop more, and are more likely to get constipated. Was she fussing with out the extra formula? Why did you add it into her bottle? She may just be full with the breast milk only and be spitting up the extra. Breastfed babies often don't eat as much as bottle fed because the bottle is so easy. At the breast they have to work for it, so they stop when they are done, with the bottle they tend to over eat some.
Bottom line, if she is not in distress, I wouldn't worry about it. If you think she is having problems with digestion, you could try another formula, or just taking the formula out all together and just sticking with BM. HTH!
Holly