What could cause

[zigzag]

Intermitent lameness on the right hind and only on a right rein? by intermitent I mean one day sound other day lame

 

[Gentle_Warrior]

saddle ?

 

[zigzag]

Not lunged with a saddle, and saddle has been checked and so has her back

 

[Smash]

Could be a poor-fitting saddle, or the muscles on that side might not be as developed as those on the other side and could be throwing him off. Another possibility is bridle lameness.

 

[Moggy]

Pulled muscle could do it. Sometimes only shows if they tweak it one day and ok the next. Get a good physio or Bowen.