Yes, call the doctor. If she has a blockage she would be sick, probably throwing up, etc. One month is a long time. Sometimes children don't go when they need to and then it becomes hard and difficult to get out. (impaction). I would call the pediatrician again. Maybe you should go to a GI doctor.
If she doesn't have a blockage---- my son fights constipation. We cut back on milk first. Then we increased fruit and fruit juice. Of course, some fiber in the diet is good too----- cereal for breakfast? But this has held off constipation for my son. He can still revert back if he does't get enough fruit/fruit juice---- or has too much milk.
good luck----- but do go to a doctor, one month without a bowel movement is alarming.
You need to go back to the doctor right away. I wouldn't attempt anything else without their help and expert opinion. If she has a blockage, that can become a medical emergency.