Hello josben,
Having an IEP doesn't mean she is more likely to get a dx, in fact it has little if anything to do with the dx process. Your dd will most likely go through an assessment process either with the paeds, an dedicated assessment centre or perhaps another professional if the paed feels its appropriate to refer her on - the actual dx process varies from area to area. She will only get a dx of anything once she has been properly assessed, if she meets the dx criteria. She won't get a dx unless they are absolutely sure.
The difficulties you describe she's having could be due to any number of reasons, or could just be down to her needing a bit more support than her peers for a while.
Its great that the school have identified the issues at such a young age and means that ,if she does have a diagnosable problem, she will be able to get all the help and support she needs to help her progress and overcome her difficulties a lot sooner than many children do - which is great in terms of positive outcomes.
IEPs are just a way of targetting specific areas of additional needs and what support the school is going to put in place to help the child meet them.
My ds2 had IEPs and 12 months of additional support following a serious illness that caused him to regress and lose skills when he was in reception year. He doesn't have or need a dx of anything, he's nt (neurotypical), he ust needed some additional help to bring his skills back up to an appropriate level. His support ended almost twelve months ago now and he's gone from the bottom of his class to the top groups this year with no additional support.
As Ellen said, hope the paed appointment comes through quickly for you. Keep reading and posting, there are some amazing people on here who can offer such a lot in terms of advice and support. 