I have a mare who is very loving, but she kicks at the gelding next to her when it is feeding time. The two of them are turned out together, and the gelding is the herd leader. When they are in the pasture, everything is great. When it comes time to feed and they are in their runs, she kicks at him through the panels. She has hurt the bottom of her hoof from doing this, and I am worried that she is going to hurt more than just her hoof. She also swings her head from side to side if you do not feed her quickly. Her behaviors start when I go to get the hay.
It sounds like your mare is food aggressive. The two clues are that she kicks at the other horse at feeding time and that she gets upset when you don't bring food quickly enough. Many food aggressive horses become so after being starved and neglected at some point in their life. If you haven't owned her all of her life, she may have been neglected by a former owner. When a horse is neglected, she begins to worry about having enough to eat. She then carefully guards any food she finds from other horses by biting them, kicking them, pinning her ears, etc. Neglected horses also worry about when their next meal is coming, so they fret when it is time for feeding. I think they're always worried that THIS time the food won't come!
Some horses become food aggressive when they're kept on an irregular schedule. They don't know when their next meal will arrive, so they start to worry about it. They may paw, swing their head, whinny or become aggressive with other horses because they're so worried about their meal. Then when you are getting their food ready, they get so excited that they don't know what to do. That excess energy and excitement may show itself when the horse bites and kicks at other horses, kicks the walls or fence, paws, spins in circles, etc.
Some horses become less food aggressive over time as they realize their meals will always come. If you haven't had your mare long, I suggest waiting to see if she settles in more over time. To help her get past her food aggression, keep her on a regular schedule. And never skip a meal! You also might try putting an empty stall/feeding pen between her and your gelding at feeding time. That may help reassure her that he's not going to steal her food, and she may stop kicking. Leave her alone while she eats, too, so that she doesn't worry that you will take her food away.
If you've had her a long time and she's still acting this way, she may always be food aggressive. While most horses get better over time, a few of them don't. They can't let go of the past and always worry about their food. For those horses, I suggest giving them space and leaving them alone while they eat. Don't go into her stall/pen, clean her stall, groom her, etc. and keep other horses away from her while she eats.
Food aggression can be frustrating because you know you aren't the one starving her or withholding meals. But you can't reason with her to reassure her that she'll always be fed. The best you can do is keep her on a fairly tight feeding schedule so she knows when to anticipate feeding time.
Good luck with your girl!