So I've known this girl for almost 5 years now and when we first met we dated for a month or 2 and got somewhat sexual but we didn't end up having anything serious. As time went on we just stayed friends and would not see each other very often, maybe once every 2 or 3 months (this went on for the following 3 years or so). Eventually she got a boyfriend and I got a girlfriend as well. A few days ago she called me to repair her computer (I'm a computer Tech) I agreed and she came by. I told her the process would take long but she insisted on staying. We talked and flirted and had good kino going (mainly initiated by her). When i finished she was very flirtatious and was very confidently telling me that I would call her the next day. She took my licence without me knowing so i called her the next day asking for my licence back and she gladly came over and returned it. Since she was giving me such good feedback I thought maybe Id try to step up the tempo a bit and through a text ask her to go for a walk with me the next day to catch up and whatnot. But her boyfriend somehow saw the text and apparently got mad at her. The next day my girlfriend tells me she got a text from her where she was asking my girlfriend to tell me to stop texting her because she loved her boyfriend. I texted her saying i wouldn't text her again. (in a chill way, not mad or anything just calm and kick back)

Why would she change her attitude so drastically and what can I try to do to make her my girl.

My first guess was she texted your girl because her bf found out and she was trying to save herself. But by the way you want to make this girl YOUR girl already, I think she's not interested and you are way too into her. You got "somewhat sexual" 5 years ago, and she flirted with you. So what? Obviously she wasn't that into you because you only lasted 2 months. Move on from this girl, leave your gf (you obviously want to find someone new) and meet other women.

Someone doesn't seem to be satisfied enough in his current relationship.
You're putting your relationship at risk for this girl.

As the guy above said, she's just saving her relationship and trying to make it look like you are the one initiating on her. I think you could've gotten laid if you escalated correctly the time she came over. Anyways, just let her be and game other girls (or keep your relationship).

Thanks
I was thinking in the same range as what you guys just mentioned but I wanted a second opinion.
Its been a few days and I'm single now (my choice)
I'm just working on my inField right now but I'm interested on learning some more in terms of how to escalate the way you are talking about @Xoved. Any good links/Suggestions