I don't quite understand her motive.....?

I have been talking with this girl for a while, we are both 21 and in college. We have gone out a good amount and hooked up a few times. The last time we went out was Friday and we had gotten together that evening,. Anyway, yesterday I had to Designate Drive for my fraternity and I picked her and 2 other girls up and I was kind of not in the best of moods but I took them where they asked and was not in the best of moods. Anyway, I was not mean or coarse with them in the car. This afternoon I texted her asking if she wanted to hang out later and she said "maybe, only if i were in a better mood" and I explained how I got a ticket last night and how I drove for 5 hours straight so that was why I was a little moody and she just responded with "Ok I gotcha" and that was it. I kind of feel like this girl is using me to begin with because of my family wealth. Anyone have any ideas how to approach this or why she is being so short with me?