I'm having a problem with my 5 year old little girl, Hope. She started Kindergarten this year and by month #2?

I thought it would be best to put her back into Pre-K again because she had not yet learned her letters and most of the numbers needed to be successful. She will be 6 in June and I still do not think she will be ready for Kindergarten. Over the past few months she has learned all but 4 letters and most of her numbers but still struggles and sometimes is confused about the letters she already has learned. I can tell her what a letter is a 100 times and the next day she still doesn't know what it is... She is very smart with other things.. like story comprehension, colors, shapes, math, organization, she has excellent behavior at school and she is a very sweet loving little girl. She can do everything that most kindergartners can do such as tie her shoes, recite her phone number and address.. rhyme... understands opposites... So, I guess my question is Is this a learning disability? How can I make her process what I'm teaching her? What type of health care professional should I take her to, to be examined? Any help on this subject will greatly be appreciated!