Why is my horse behaving this way?

So I have just started leasing a horse in May. She is half arabian and is 13 years old. Her owner is struggling with drugs and illnesses. He never really rode her but had her fully trained. He boarded her at my best friend's stable's where I first met her. Now, he found me to lease her! We started out small, just daily grooming and whatnot. When my friend rode her horse, I walked (Bytheway her name is Spirit) Spirit with a lead rope. Because of the fact that she was never really loved by anyone, she was unsure and jumpy and very herd bound. Eventually, she fully got over all her fears and has since turned into a nice little mare..very sweet and very loving. I started to ride her bareback and she loved it! My friend and I rode all thorughout the woods, with little jumps and bolts here and there because Spirit is a little skittish. Now here is the problem! Yesterday, we took them out as usual and rode fine, until we reached one of the many openings to the creek. Here, Spirit decided to act up and tried to turn around to go to the creek. Since I started riding her, I noticed that every time we go by this opening (one of many!) she tries a little harder each time to turn and go there. Yesterday she spun around and took me into the creek. Yes it was hot and humid, but we had already played in the water in a different area...the area where we have always swam. Once in the creek, she just stood there. After a few, I turned her and she happily went about back to the barn, where she got a good bath. Now today...Oh boy! As we got to the opening, my friend said that we should take another trail away from the creek. Before I could turn her away, she kicked up into a trot and went down to the creek...off road! Despite my great ability to handle horses, Spirit just acted as if nobody was on her back. She would turn in the direction of the creek, trot there, stop when I screamed whoa and pulled and then freeze. When I turned her away, she spun in tight circles and trotted to the creek. I tried my best to stop her because she was heading towards the creek without realizing that there was a large hill with only smooth flat boulders that were slippery. She ended up sliding down the hill and right to the creek, where she once again stood. I had a whip with me that I don't usually use but she did not respond to that either.I was very, very annoyed and she had to know I was upset from all the shouting and whips. She has no aggression in her at all, just extreme stubbornness that came out of nowhere. A while later, we were cantering through a field as usual when she suddenly stopped and wheeled us into a bush. I ended up jumping off and walking her back.( I know I shouldn't have, but I really had to as she was acting up too much...and I did not have my whip with me at that point) After our terrible ride, we, now I don't know if this was correct, lunged her hard. Later that day, we rode again. She was much better, but by that opening, jeez she just tried so hard to get to the creek. Luckily I was as determined to get her to go as she was to go stand in the creek and we had a nicer ride. So I guess I want to know why she suddenly has so much determination to go stand in some shallow water after she already had a swim. It was not too hot today, so she's not too hot. She has a curled hoof problem that was temporarily fixed from the farrier just recently, but maybe she is sore again and the water feels good? I really have no idea! She was so good before at riding and now since yesterday is just being very ignorant and stubborn. We have never even swam in that area before! Never! And there are many opening to the creek, but she has decided to get stuck on this one. She is not in heat. Does anyone have any ideas on what her problem may be? Do you think she is trying to test the dominence in our relationship? I have never had her disrespect me, until yesterday, we had a very firm and good bond. I just want to be comfortable with her. This is just so out of the blue...I am really concerned.