Recently our daughter has decided that she does not like other kids at all. When at the various playgrounds she now stays next to us and will not climb into the slides. The more kids, the more timid she gets. After we leave she says to us "I dont like them!". We have always been in the playrooms with her and we dont know of any incident other than unattended boisterous boys which we think may have intimidated her. At the pool she's better.
It sounds like your daughter may be getting overwhelmed at the playground. For a while you may want to focus her plans around one on one playdates in calmer environments. When you do take her to the park, choose parks that have segregated toddler areas with smaller slides and younger children. Try to help her to verbalize what it is she does not like so you can validate her feelings and help her to problem solve.