Is she stalled or put out in a small paddock?
My mom’s gelding will swell up when he’s put into a smaller paddock in the summer months, but when he’s out to pasture 24/7 he doesn’t. For him it’s just another form of stocking up, only higher.
Have you looked into your grain? is there soy in it? I’ve never seen soy in grain before.
Does she have a blockage in her udder? My aunts horse had this happen to her, after her baby was weaned from her. It would get hard and warm with no discharge, but once cleaned(similar to a sheath) it went away.
Is she getting any other type of manufactured food product? Treats?
I would start with food products first and move your way down the line.
"Take the time it takes, so that it takes less time."
"Expect a lot, accept a little, reward often."