My bright, able son didn't like Montessori. This was after the area's regional Montessori director (a friend of my sister's) saw him at my sister's house and said glowingly, "He's real Montessori material!" He just did not like it at all. After a while, there was no point in sending him there, his behavior was getting more stressed and anxious, not less. He is doing great at a different preschool now.
Maybe the way Montessori is structured is bothering her. Maybe there has been a change of student body and some friends have left. Maybe she thinks she is being sent away so Mommy can finally get some work done. For whatever reason, I would re-consider the school. A lot of people prefer Montessori for the supposed prestige and because it is thought to be a great prep for elementary school, but for some kids, it just is not the right school.
I suspect you are dealing with anxiety (probably social anxiety). If so, this is an inherited dispositional trait and not caused from life experiences (re school, bus, teacher, parents, etc.). If social anxiety is the issue, I suspect your daughter only knows that she is very "uncomfortable" at school but unable to reiterate why. Probably she feels safe and comfortable and in control at home, and thus the reason for not wanting to go to school. As I stated before, if anxiety is the issue, these fears/anxieties are innate - comprised of physical, emotional and mental aspects. Those with anxiety are unable to control their perceived fears without proper treatment - that being said, anxiety is very common, highly treatable with an excellent prognosis.
I might suggest you google the phrase "school and anxiety" or "social anxiety in young children" or "behaviors of anxiety in children" or similar words/phrases to read more about this issue. If you believe that anxiety might be the issue, then speak to your daughter's physician. If he/she is unable to help you, then ask for a referral to a medical mental health specialist with experience in anxiety disorders (as a child neurologist or child psychiatrist). Concerning anxiety, be aware that this will not go away nor will your daughter outgrow it (there is no cure); however, anxiety can be successfully managed with proper treatment. All the best ...