It depends on what her usual pattern is. If she usually goes every day, then she might be a bit bunged up. Have you noticed any other problems - crying and straining, off her bottles or anything like that?
If you think she is constipated try giving her a little cool boiled water - 1/2-1 ounce per day will help things get moving. It can take a day or two to have much effect though, and if she seems to need it, try to give it every day.