You can’t definitively say your horse’s behavior is not pain related. We’re talking about a stoic animal with a complex self preservation mechanism and intermeshing of behavioral and emotional responses.
Does it hurt now? Maybe not but it might have at one time. Maybe they’re worried about pain even if they aren’t currently feeling it. Maybe it only hurts sometimes. Maybe the bit or saddle only pinch at certain times or in certain movements. Maybe the pain always exists but they can deal with it, until you stack one stressor too many and that’s when they blow up.
Until horses can talk and tell us they don’t feel pain, we can never rule it out, and have to approach the situation holistically – as behavior and pain are incredibly interconnected.