Whenever you switch a baby from breast milk to formula, constipation becomes a concern because formula does not move as quickly through your baby's system. Switching between formulas can make your baby constipated as well.
I remember once, when my step son was visiting with his new baby, he ran out of formula. My eldest son was a baby at the time and so he just used ours , but they were different brands. His baby became constipated and he literally had to pull the impacted poop out of him. Constipation is very dangerous.
While you are trying to find a formula that agrees with your child, I suggest you use this baby constipation remedy. Since this natural product also works for adults, it makes sense to have it on hand in your medicine cabinet.
Since your baby is plugged up right now, I also suggest you try some pear juice. Simply mix 1 ounce of pear juice with water in a 2 ounce bottle. If this does not clear up the constipation, take your baby to the pediatrician.
Find a formula that works and still with it. Better still, return to breastfeeding.