Agree with the comments. Vet check is a good idea, and always tighten the girth one hole at a time. Also recommend checking saddle fit. Horses’ bodies change, and often they tolerate ill-fitting saddles for a long time. They can get to the point where they are so sore that they can’t take it anymore and express their pain when the girth is tightened. Also, if you have a massage therapist that you trust, a treatment may be in order. Your mare may need an adjustment. Good luck!