Pooping problems

My daughter has been having horrible pooping problems... This may be TMI, but I wanted to know if anyone has gone thru the same thing. She is a preemie 15 months, 12 corrected if that matters any... Anyways, she has been having trouble pooping for about 2-3 months now. When she poops, it's about every other day or so, but her poop is very hard and HUGE! It's like a grown man poop. It rips her butthole and it bleeds every time! I took her to the dr and all they did was tell me to give her miralax. That helped once but hasn't helped since. She takes 1/2 a table spoon once a day. I called to set up another appointment and they just told me to continue the miralax. It's not helping! Is there a specialist doctor or something I can go see for this? It's ridiculous that she has to go thru this pain! She won't walk or move for that matter when she has to poop, she just sits there whining and clinching. I've tried EVERY other natural laxative nothing is working! I feel so bad for her and helpless because I don't know what is wrong and don't know what to do! Any advice?