Is it normal for a baby to have constipation after introducing mixed grain cereal?

Yes. It can happen with thickened formulas too. Try addding a SMALL amount of prune juice to her cereal. That should get things moving a little. Those little muscles just need to learn how to deal with something a bit more solid than milk!

My 7 month old gets constipated from time to time as well. I just give her pears or applesauce and she is better very soon. I always feel so bad when she is in that predicament.

Any change in food can be a problem with babies of any kind, even in animals.
Recommend you get child to a doctor quick, you could loose her.

Yes it is common that an infant or small child will become constipated with a sudden change in diet. Neverthless you should treat it as constipation and not worry so much as to how it happened. Constipation is constipation.

Quoted from my source:

In addition, a lack of fluids can cause a child’s stools to become hard and dry, increasing the risk of constipation. Sometimes, a sudden change in diet will contribute to a child’s constipation. For example, it is not unusual for infants to become constipated when cereal, other solid foods or cow’s milk are added to their diet.

The most common causes of constipation in children are related to diet and a lack of exercise. Low-fiber diets often contribute to a child’s constipation. Fiber in a diet (e.g., fresh vegetables, fruits or whole grains) is not digested by the body and passes through the intestines virtually unchanged.

/quote

Feed your child plenty of fluids and especially baby foods / fruit juices. In particular to the fruit juices, mushed prunes and prune juice would be ideal.

Check with your doctor, I wouldn't think the mixed grain cereal would be blocking her up. It has more fiber than what she has been on so if anything it should be going through faster. Although maybe it's taking her a little longer to digest it.

If she is really constipated, a little fruit or fruit juice added to the cereal will help. Also, if she's on formula, try a little Karo syrup added to each bottle, a teaspoon or so, that seems to help. Formula will sometimes plug them up a little. My son used to get constipated too, I would give him prune juice and then we would stay home the next day and wait for it to clear him out. Messy, but it always worked.

Another possibility is that your daughter may be having a growth spurt and actually needs more to eat. She may be digesting it all and doesn't have enough left for ****. This happened with my sister-in-law's baby, she thought he was constipated too but he was going through a growth spurt and needed more to eat. After that he started pooping again. He had been very fussy and they thought it was because he had cramps from being blocked up, but he was really just hungry!

Good luck!

cereal do not cause constipation, maybe your daughter is not drinking enough fluids.
Try offering her water more often.

Yes, but check a specialist medical baby site for more information, try using google.