Hello,
Possible side effects of too high intake of any osmotic, such as Miralax, include diarrhea, thirst, and possibly even confusion. Hence it is advisable not to take Miralax when you have diarrhea.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Usually Miralax is used to make you go to the bathroom. If you have diarrhea, you should probably stop taking it for a while and see what happens. Your digestive system should get back to normal once you stop taking the laxatives.
This happened to my sister too, and we called the doctor for her after three days because the diarrhea didn't stop even though she wasn't on the medication anymore. I would suggest you do the same. If things don't firm up, call the doc so you can avoid severe dehydration.
Best wishes,
~ Em