If after a while you'll have to think about the potential answer to this question. When you ask you must think about what you'll do if she says "I don't, or won't give head" there women who just don't like to do this act. At that point you can bring it up at a random time, not during and certainly not after. Just ask her kindly, I just want to know... "is there a reason that you do not go down on me?" She may just be nervous or scared or she be the girl who doesn't give out head- ever.
Edit: I don't give head so soon either. My man waited 6 plus months before I gave head it's something I give only when I'm comfortable sexually and deeply emotionally involved, and that doesn't happen in 2 months.