Kicks in meaty areas

After long years of ownership I am now at a loss with my first kicking incident that has shown up. I noticed my mare had a mark on the meaty part of her chest a few days ago. Mare is very accident prone to rubbing and whacking various body parts. Didn’t think much of it and went about my day. Haven’t been able to ride or work horses in a few days but last night while free lunging in the round pen I noticed mare is now off going one way. Very intermittent. She was also feeling spicy in the cool weather so head snaking and playing like crazy. What I noticed was head up like way up showed a head nod and off step. Head down long and low looked totally sound. Looked fine while cantering. I am thinking that head up while going the one way must stretch a muscle that is currently tender?