> 
> ところが、引き返す**となったとたん**、痛みがましてきた。  
> 
>  However, just as it became *time to* go back, the pain increased still further.
> 
> 
> 

I don't understand the grammar of the part in bold. My translation is a best guess. 

I'm familiar with the use of とたん to mean 'just as', and なる to mean 'become', but I don't know any grammar where と+なる can attach to a verb.