You can always try another bit, tho I would wonder why all of a sudden, a bit that had been working just fine before would not work now. Of course, looking
At the saddle fit is also a good idea.
Maybe, just maybe, you are riding differently? Are, perhaps you have done so much work on collection that the horse is souring to it? You don't want to lose the "forward" mentality by too much work on the head. That might be what's happening, you are getting stronger in your asking of the horse to flex.
I am just wondering about different possibilitys but the first that always comes to mind is a physical issue.