I need to know

Ok so the horse I have my lessons on is an 6yr old ottb that was retired in August.

So I have been riding him every 2nd week and he came up lame in the start of December. They had the bodyworker out and she says that I'm unbalanced and sitting wrong in the saddle. She says that my stirrups must be longer but it's literally on the second last hole ( I'm 180cm and he's 17hh). So I just want to know if it was possibly my fault that he got hurt or not.

EDIT: there is one other girl that started riding him in November (she rides him ones every week)

He hurt his stuffle according to the bodyworker