This is demonstrating the effects of propaganda and the satirical nature of the entire book. Clover's mistrust of her own memory is coming from habitually being told lies. When people are lied to enough, they start to believe them, especially if the lies are so closely related to the truth. Every time a commandment is changed... it is just a few words that are added or affected.
Squealer does a great job being a reinforcer of propaganda as he asks questions that challenge the other animals to question their own beliefs of the past. Squealer effectively provides arguments too about why things need to be the way they are, for the pigs mental benefit in most scenarios.
The answer to this is not actually spelled out in the text of the book so we have to infer what the reason is. In my opinion, Clover does not trust her own memory because of a couple of things.
First, she has been sort of influenced to think that Napoleon and the leadership is always right. So that might make her think maybe she is wrong about her memory.
Second, she herself can't read, so maybe that keeps her from being confident.
Finally, I would say it is because the pigs keep changing things. The other animals must have the sense that they aren't sure which of their memories are and aren't accurate. This would also cause confusion for Clover.
In the book "Animal Farm" the pigs have become controllers of the other animals on the farm. Napoleon rules the farm with his vicious dogs at his side and Squealer speaks on his behalf giving out orders. The rules they had first formed as commandments appear to be broken one by one. The dreams of having an operative farm free from human involvement has begun to fade away.
The animals work like slaves. THe work is supposed to be voluntary but if one does not work the 60 hour week one receives no rations of food. Clover is the voice of reason and always reminds Boxer not to work too hard and strain himself.
The reason that the naimalss can not remember the exact wording of the commandment is because not all was written down. This allows Squealer to manipulate the animals.
The bed law was written on a wall but Clover can not read more than a few letters so she had to ask another to read it to her.
"Have you any record of such a resolution?"(64)