Hello and Thank you for your question,
Three days of no bowel movement technically does not constitute a definition of constipation, however, to avoid further problems you can do one of several things.
You can increase the dose of her Miralax to twice or three times daily for a few days. Or you can purchase a bottle of Magnesium Citrate (available over-the-counter) and have her drink the bottle. This is a laxative so she will definitely have a bowel movement. You can also give her one or two tablets of Dulcolex which is a strong laxative, however, this may cause her to have abdominal cramps during a bowel movement and may make feel very uncomfortable.
So I would use either an increased dose of Miralax or 1 bottle of Magnesium Citrate to relieve current episode. If this continues, her dose of Miralax will need to be increased permanently or she may need to be switched to a different medication if Miralax fails to work.
I hope you find this information helpful. Please feel free to follow-up with any other questions you may have.
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Best wishes and good luck,