I'm amazed that you share a field and have no agreement on feeding. You are both at fault for not sorting it out before. You cannot expect someone to wait for you to bring your horse in (and I know a lot of people would be really angry if someone fed their horse). She should have told you at roughly what time she would be feeding her horses so you could be there before or as you say you could have left some for your horse. You need to talk not expect each other to know what you want. Sorry but I really cannot understand how people can say they understand you having a rant
our YO would give a major boll*cking to anyone putting hay out in a communal field (she gets cross enough with people putting it in individual paddocks).
Asking for trouble imo - don't think you are over-reacting at all!