I had same situation in beginning and, hell, it was worth waiting. Weeks, to about a month -maximum, is alright. When she is a non-virgin and makes you wait much longer, then do worry. In my case, when we started having sex, it turned out great, and even better with time.
To me, it's a good sign if she doesn't put sex as priority straight away, and first/second date fucking is just a red flag in my opinion. Depends on a full character. BUT - I judged my girlfriend as shy, even classy and cautious as well, however, beware and follow this: the problem was that she had first and only sexual experience outside "commitment", in a drunken messy situation, a one night stand. And felt the need to tell me "but I assure you there was no emotions or anything like that at all" or that I am better, like I should make sense out of that statement anyhow. I am, well, very sure she is proud of it. When I dug, I knew she doesn't have any regrets. It was really easy to find out, obvious in her 'coded', obscured speech on fb wall with her friends. Furthermore, I am sure she's highly sexual, and notices other attractive men. That in itself isn't a problem, but take MY advice: everything that happens early on, all the small things you notice, will matter later on, even if you ignore them, they will serve to paint a big picture in the end of the day.
And I have the problem at the moment, doubts about her potential drunk situations, doubts about why she made me wait but had tricky ONS in past, gave it right away to loose virginity to a stranger or strangers, doubts whenever I get turned down for sex, doubts if she's telling the truth, etc. You will be in exact same situation if it goes in direction similar to mine.
So, you can't know her reasons at the moment, they may be valid and fit her character, but perhaps she is "shy" and cautious with you. I tend to think that most of my relationships were girls compensations until they met someone better. Fuck it, but you'll have to do harsh analysis here and trust the test of time. Won't be easy if you really care about having a real deal, not some useless so called relationship experience. This never is easy, because you could find pearls or crap.
And you'll have to use everything you lived through, everything which built you, your personal insight, to correctly judge to whom you open the doors into your life.
Tell me more, did your girlfriend mentioned her past?