Get her back checked it may just be that you sit in a different position while you flat work and are therefore putting more weight on a sensetive part of her back
Keep her on her toes, do random things every once in a while so she doesnt know what’s coming up next
he may have hind gut ulcers or gastric ulcers.
I agree 100% with Meili G and also you can do haunches in haunches out walk to canter lead changes turn on the fore hand anything like that
Do different things to keep your horses’ mind active. Circles and workin on bend is great. You can use poles to lengthen or collect your horse. You can create a jumping course but lower the poles to the ground so you can work on your bend and turns, collection/extension, and distances. Raised cavaletti also make your horse work on lifting their legs. If you used raise cavaletti your horse can’t get bored/lazy. You can also work on lead changes.
Well, Lola used to do this and I realized she was getting bored. What I started doing was randomly queing her left or right. Or I'll suddenly ask for a half-pass. It caused her to be unsure of when I was going to que her. I also incorperated more trot poles amd would space them weird at times, so she would have to pay attention more to where her hooves where. If that doesn't work, I sometimes just had to bear through it, and just keep asking her forward!😂 Hope this helps!🙃