I wouldn't say ulcers just because she's pinning her ears at feeding time. Some horses get food aggressive...
There are many other symtoms for ulcers, like losing body condition, being grumpy while groomed and or ridden, etc..... If your horse is simply pinning her ears while you are feeding, she is just lacking a little respect for you. You can't stop her from pinning her ears, but just make sure she doesn't take any steps towards you or anything else "aggressive" like that.
She can pin her ears all day as long as she respects your space, and that you say when she gets her food.
I also think your second change of not riding as much ties in.... you aren't handling her as much and she thinks she is more alpha.