Try putting pressure on the places where she gets really angry whenever you groom there. If she reacts to the pressure, it could be that there is something that pains her in that spot. If so, then you should definitely get a vet or a chiropractor (depending on where she is in pain) to examine what could possibly be going on. If she is not upset by your pressure, then it could be that she is in season or an emotional issue.
Probably hormones. My mare is grumpy the week before she's in season and the week she's in season. If you've had her a long time and it's only started then it's more likely to be pain. Though ovary problems can start at any age.
That happens to my green mare alot. When it happens I will usually work for about 45 minutes with her on the crossties just touching her and working my way from her poll to hindquarters. She's usually fine after that! 😁😁
It could be that she is going into heat? It typically starts in early spring, so depending where you are, and on your individual mare, this could just be a temporary thing. Some mares don't act any differently, some become a little over the top while they are in heat. I don't know how long you've had her, but if you haven't had her over a year, and haven't had her go into heat before, this might be part of that process.
Usually when you have sudden aggression in any animal is means something is off with their health. Check her coat for rain rot or anything else that could be causing her discomfort.
Definitely check and see if any area is particularly sensitive.
I would definitely consult with a vet if this is new, as usually that sort of behaviour is your horse's way of telling you that they are uncomfortable, and something is wrong.
It could be hundreds of things! Try and see if it is central around one point- her belly, neck, hips, back, etc. I would probably have a vet, masseuse, and/or chiropractor have a look at her, depending on the area.
Mine is doing the same thing, she loves a good scratch on the neck and that's about it.
If it extends to riding, I would check with a vet to make sure it's not something like ulcers that are bothering her :)
She could just be in heat. I would not be worried unless it goes on for a long time! She is just being a mare :)