Sudden limping in a child could result from a number of
causes, some
of which may not be obviously visible and may be associated with
pain referred to other regions. A few causes include infections/ inflammations, fractures, growths/ masses, Legg-Calve-Perthes Disease., neural/ muscular disorders, a few blood disorders etc. I would suggest getting this evaluated by an orthopedician for an accurate diagnosis and appropriate management.
Hope this was useful.
Take care!