Jing Shao was...probably too rough at that time(he was still his angsty self/hasn't gone back yet at that point), and it doesn't help that Jun Qing's body wasn't the healthiest to begin with. It's not that he's too frail, it's that he's not physically as strong as the average man, but he's still a bit stronger than women though. Also in the novel, the empress deliberately let him kneel for far too long, adding more strain to his already sick body at that time.