This could be indicative of a high, hind end lameness situation. Watching your horse and how he carrys/sets his tail can be a good diagnostic tool, and was for me on the racetrack. Sometimes the most subtle changes can indicate something might be wrong and your horse is altering his/her weight to compensate. You are very in tune with your horse. It is most important to have your veterinarian do a complete physical exam and share with him these abnormal characteristics. Thanks and good luck.