Does being tongue-tied lead to stammering?
Please clarify the myth that being tongue-tied can lead to stammering - no. The proof is your own daughter who according to you was very clear when she was younger. Had her problem been due to tongue tie she would have never been clear at the first place.
Stammering is due to a number of reasons which are mostly psychological in nature but vary greatly from person to person. Please get her assessed by a qualified speech therapist at the earliest before her problem becomes worse. Its only after her assessment, treatment can be discussed.