I’ve included replies from both Scotsbadboy and Pearlsasinger because it’s very, very important to understand that what Scotsbadboy is suggesting could get you killed, and what Pearlsasinger is recommending won’t.
If it’s towards the end of a ride it’s often an exhibition of tiredness. I often lengthen the reins at the end of a ride for the last 5/10 mins to allow the horse to stretch their neck out and start to relax and stretch out their muscles.
If she’s medicated and sound ride her, leaving her for months on end will do her no good whatsoever (as said above). If there are times when you think she’s not 100%, then it might be time for a veterinary review. How do yo judge if she’s not ‘right’?