It is really hard to know if it is due to her age or if a learning disability will show once she is in school. Right now, don't worry too much. When she is in kindergarten keep a close eye on her progress in reading. There is a list of dolch words that she should learn (you could find the list on the internet) if she is not learning the words you may want to have her evaluated. At that time, the school will tell you that she is too young, do not believe them. Research and hire an advocate if you can.
My daughter has dyslexia. Like you, we have read to her since she was an infant. She loves books. When she was a toddler we would read a book to her once and she would have it memorized. In kindergarten she could not learn her site words. In first grade she continued to memorize books but still had a difficult time learning to spell, she also displayed a list of other symptoms related to dyslexia. I requested that she be tested, the school told me she was too young. Not true, the younger it is diagnosed and intervention started the better the outcome. I fought it, got her tested and just as I suspected she had dyslexia with an IQ of around 124 in 1st grade. She is now in 3rd grade, loves school, still loves books, and reads at a 4th grade level because she is in a reading rich environment, she is smart, motivated and we provided appropriate intervention at an early age. She does reverse her letters sometimes. Every now and then does she write entire words backwards, not often.
So, don't worry right now, just keep an eye on things and if you are on top of it, everything will be fine. Often children with dyslexia are extremely intelligent and possess many gifts. If you are curious, google the National dyslexia association for a list of symptoms to look for, I think it breaks symptoms down by age.
Good Luck, it sounds like you have a very smart little girl.
p.s. I have heard that a lot of kids who do not have dyslexia or any other learning disability do what you describe when they are young, I just wanted to tell you my experience so the you will be aware, not to worry you.