I'm sorry, I have nothing to add, beyond that a visit to the vet is the only way you can know for sure. A few years ago I had a Heb ewe who looked as huge post lambing as she had before. One side was larger than the other though. I never did discover why she was like that, but she lived for years, and lambed several times

But your Soay is worse than she was. One way to find out if it's fluid or air is to place one hand on the swelling, then tap with the other, all over. If air, it will sound hollow, otherwise not. You could also try pushing the belly from side to side and listen for sloshing, and put your ear on her side and listen for normal bowel sounds.
Don't forget that sheep are prey animals and will look well sometimes until the last moment. I hope it's nothing.