^^ Agreed. Changing her to a hay only diet for a short while to see if the symptoms go away would be a good start, then adding in any other feed or supplements one at a time.
Is she just getting one flake of hay in the morning and one in the evening? How long does it her to eat it? Does she have grazing throughout the day? Unless they're really huge flakes or she's on pasture, I suspect she's going way too long without food going through her system, which is a major reason for ulcers. Horses are designed to eat pretty much constantly: their stomachs don't stop producing acid when they stop eating (like humans do) so when they don't have food the stomach just gets more and more acidic. Putting the hay in a slow feeder net will make it last longer, but I suspect you may need to feed more hay as well.