It is normal for babies to take long without pooping. During such period, you will often find that the baby is relieving herself more by urinating than her normal times. However, if you suspect that your baby could be sick, it would be good that you have a doctor examine her.
You can mix 1/4 teaspoon Dark Karo Syrup (not the Light) to an 8 ounce bottle of formula and feed him that. It might be the Iron in the formula that is the issue, I know it was with my daughter. Good luck!
My daughter always went a week between poops. she was also breast fed and formula fed. I was told its ok until after 7 days. She never went more than that long. But, some tricks I was told is to lightly put a rectal thermometer in, you won't have to go far... And she will poop. Also, you can buy baby suppositories. Ask the pharmacy before doing it. I was told to cut it in half and then cut a little more off. Warning... Wear gloves. I was told it would be instantaneous.