Laurie??

Hey wanted to ask you a question.
I got Tina in the round pen tonight and man did she show her butt...The new pen is about 80ft i think? It's HUGE. Anyway, got the longe line on her and started working her. About 5 mins in to it she started showing her butt by kicking out (she was no where near me) bucking and thrashing. I then whoa'ed and her and jerked (?) her to a stop, raised my voice and then started in the oppisite direction. About 5 mins in to it she started again, but i didnt stop her that time just clicked her up a notch. Whoa'ed her, called her and she came with her head down...loved her a bit and then off in the other direction...same routine, butt up, leg up..acting all kinds of crazy. I worked her about 30 minutes, she worked up one heck of a sweat. After that, i took the line off of her and she followed me around the pen for about 10 minutes..i left her in the pen to get the lead rope and squeegee. I then walked her around and then hosed her down.
My question is WHAT got in to her? Shes never acted like that before. Is it because she from a small round pen to a big round pen? In this one she can actually extend and run. PMSing?
Is this normal behavior? I think she would've "won" if i would've gotten scared and let go of the line, which i didnt. Of course with her being 16 hds high, i just knew i was going to go flying through the air when i pulled her.
Any suggestions or comments?
Jen