It sounds very mare-like to me!
My mare came back the same way. She had not had any fights with anybody when with the stallion, but before I sent her there, she liked to boss the small ones around, and when she came back, she began bossing the rest around as well.
She's not the alpha mare yet (she's only three years old), but I'm pretty sure she will be in the future.
And I'm also sure that her temper has changed due to the pregnancy. Another mare did exactly the same thing last year. She even changed her temper towards people. Both mares have changed from the day they got pregnant.
And I'm pretty sure that's what happened to your mare as well.