What do you guys think about a girl stiffening up when you touch her

So this happened about a month ago and I chatted up this chick and she asks me when my birthday was, I tell her the day of my birth lol and she says we're born around the same time.

I then reply with oh hey looks like we're the same sign (I say the name of the sign that we both are).

after saying that I bring her for a one-handed hug with my left arm. I'm not looking at her, I think was looking forward or  away (lol I don't know why, I usually don't do that but I did this time). When I do that I feel her body stiffen up. Imagine giving a cardboard a one-armed hug lol.

Her and I were walking to an event going on at the school. 

We talk here and there at the event. You need tickets for the event and the more tickets you have the more chances you have for the raffle of about 6 different item(s).

i run into her at some candy cane game. The game, you use your candy cane (picking it up from your mouth lol) and you use that cane to put about 7 of the candy canes in a bowl. I was having a tough time with that game. When I first get there I chat up the girl that's like officiating/giving the tickets. It's a fun and playful vibe towards each other (she was teaching me how to play the game, I kept cheating). 

Now the girl that came with comes by with her friend (i think she's one of her Chick best friends, this is the middle of the event, time wise) and I kind of observe them.

She loses to her best friend and she felt some sort of way about it with her. She (the girl I came with) in my vicinity, and she says as she's pulling up her pants some, "is my butt showing?" With a sort of smile. Looking back at that I thought it was funny she say that. 

I give a random/surprised look (hopefully not a disgust look because I wasn't haha). I don't say anything, just a look about mid way through I finally realized she was looking at me the whole time, it's just us (her friend, and chick ref but didn't say anything, I think they knew what was going on). 

Her: whaaat? Don't give me that look (says in the most feminine way, plus a smile)

Me: (if I remember correctly, this is what I say and do) "So let's play a quick game" *reaches over, gets a bit closer and touch her lower back you know right above the right ass cheek haha* 

she does it again, her body stiffens up, I think she's the one that looks away this time. 

We play a game (I cheat, can't get the mechanics of the game down lol). After the first we play several more, she kept saying, I want to keep playing you because I want more tickets :)) 

You beat someone at the game you get the tickets, she never beat her best friend, but always beat me. The chick official for that candy cane game gave me some even though she wasn't supposed to. 

We go to put our student info on the tickets for the raffle. We have to put it on every single one of our tickets lol. As we're doing this she asks/tells me: can you do this for me?

Me: nope I'm not your errand boy (or something to that affect)

I jack some of her tickets. 

She gets a little competitive with me (like a girl in the school yard) I worked hard to beat you for those tickets. 

She would say this as if I cared. It turns out I grabbed the tickets that didn't matter the b-sides that you don't put info on there just extras lol. 

I tell her this, she doesn't believe me.

So I repeat myself: I grabbed the wrong tickets, these won't be part of the raffle after a small pause I wink right at her. She continues to hold my gaze and gives me and innocent type look, says as she's looking away : alright then.

We talk some more here and there, I don't escalate on her past that (no number or anything). I was upset with myself on the drive back home. As of currently I have her on Facebook. 

I was wondering about why she would stiffen up when I touch her. Some thoughts?