the term you are searching for is 'aroused'. not 'loosened up'.
if your girlfriend is not sufficiently aroused before having sex then this can be painful. it is also possible that she does not produce enough lubrication during sex, and the friction is hurting her. if this is the issue, then investing in a tube of lubricant should solve this issue.
is she specific about where it hurts during sex? deep inside? her vaginal opening? her clitoris?
if she hurts deep inside then it is likely your penis is hitting her cervix. if her vaginal opening hurts this could be caused by rough sex or by a lack of lubrication. if her clitoris hurts during sex then she must have a very sensitive one. it sounds strange, but very occasionally when me and my boyfriend have sex my clitoris might get sore because his big wirey pubes are rubbing against it! it doesn't happen often, but if she is getting clitoral pain during sex, this could be a possible cause.
you always have to bear in mind just how sensitive the clotoris is. some studies have suggested it may be 14 times more sensitive than the head of a penis. when you are going down on her be .e.x.t.r.e.m.e.l.y gentle. more so than you have ever been. don't go harder and faster when you think she is going to reach orgasm, just be gentle and tender. instead of stiffening your tongue, use it flat. if you want to use the tip use it really carefully and softly because it could cause pain if you flick it over the clitoris. in short, just try changing techniques a little.
as for the vibrator question, i say go for it. in my opinion, if anything is more likely to bring her to orgasm it is a vibrator. you can get fairly inoffensive small ones that aren't going to scare you or her!
if you would like any more help feel free to pm me.