My EMS pony was exactly the same. It was the only sign, other than being porky, that he had!My EMS pony suffers from a swollen sheath when his lymphatic system is compromised due to much sugar so if your pony is on grass it could be the cause. Unfortunately the grass is like rocket fuel at the moment and even if you think there isn’t much out there there is and it really doesn’t take much to tip them over the edge.
With mine I remove him from all grass and put him on a mud patch and when he was ridden a good fast hour sorted it out by kick starting the lymph system.