Answer

By Your Horse

Horses tend to show aggression when they’re in pain or have a fear of pain. Sometimes, even when the pain or fear has been taken away, they still react out of habit, eg when girthing up some horses still nip even though you’ve girthed up gently for ages.
You are right to be on your guard, especially when handling your mare from the ground. When rugging and tacking her up, always make sure you stand in a safe place. Tie your horse up so she knows exactly where her place is – and you know where she is. Calm discipline and reward will help tremendously.
Is this behaviour related to when your mare is in season? You may notice a discharge from under her tail. She may lift it and squirt and show more interest in other horses, or even be more aware of her own natural space. When you notice these changes, mark the dates in your diary and see if this coincides with your horse’s more aggressive behaviour.
With the lungeing, I suggest you have a lesson to be taught how to lunge with two lunge lines. This technique will help you to keep better control of your mare’s body and keep her on track. Lungeing with two lines isn’t easy, so you’ll need to be shown the way and then practise before trying it with your mare.