Toddler constipation/diarrhea

My 20 month old has had chronic constipation for the past 6 months and the peadiatrician had us try miralax again (we tried it when this first started and it didn’t do anything for her) as a last resort before seeing GI doctor. Well I gave it to her a week ago and the next day she had a normal soft poop so I stopped giving it to her. But ever since then she has had blow out diarrhea that smells foul and is very dark green and has lots of seedy texture to it. I only gave her the miralax that one time and the only thing I’ve changed in her diet is that she is having almond milk in her oatmeal for breakfast instead of whole milk now. Other than that she eats the same thing she has been the last 6 months which is lots of berries, chicken and pastas. So now I’m just confused what this diarrhea could be coming from. It’s like she went from one extreme of horrible constipation to now having blowout diarrhea every other day. Any advice appreciated!