"B" is for Boobies, thats good enough for me
It could be a number of things. Maybe she's menstrating. Maybe she felt like you took advantage of her when she was drunk. Maybe you just didn't have her hot and bothered enough.
Anyway, here's what I like to do when I feel like putting some nipple in my mouth.
Step one: Make out. This doesn't really need explaining, as it is easy enough, and has been covered enough in other posts.
Step two: Get closer. If my girl isn't close enough (I like her to be on my lap with her legs wrapped around me, mmm), I just grab her hips and pull her closer. If my member's happy, I want to make sure she feels it.
Step three: Intensify the kisses, and make them more passionate. Nibble her earlobes, wrists, inner elbows, neck, be sure that all the while you're exploring her with your hands (stay short of the breasts and vagina for now, however, feel free to traverse the inner leg for a bit). Be sure to rub your fingers through her hair. A very little amount of playful tickling works well here.
Step four: Make her wet and perform "saftey checks." Here are a few things you can do to make sure you don't get shot down when you try to get the shirt off, while arousing her at the same time. Rub the inner legs more, still staying short of the vagina, but closer than step three. If she pulls her hips away, you aren't there yet. Also, rub her butt a bit. Watch her body language. While doing this you might want to pull them apart a little, dont like grab each cheek in a death grip and stretch, though. Just push together a little and bring your hands towards her stomach. I can't remember who said it, but someone posted saying that it spreads the lips of the vagina a little, which can be stimulating. It was some guy who talked about hoes a lot and had a very "gangster" writing style. If I remember who it was I'll edit this for them. Another thing to do is to put your hands on her hips and slide your hands upwards, moving towards her back just after the ball of your palm graces the outmost portion of her breasts. Make it subtle so its not obvious that you're meaning to feel them. Once again, watch body language.
If she responds positively perform them again, except traverse slightly more of her breast and inch closer to her vagina. Don't go full circle and grab them just yet, though.
Step five: If she responded well, take her shirt off. Don't ask, just do it. Slide your hands down to her hips, grab the sides of the shirt, and pull it up. If you aren't sure yet, keep doing step four. Try the breast thing with your hands under her shirt but above the bra.
Step six: Basically keep doing step four, but be sure to regress back to step three now and then to keep the kissing interesting. Finally, you may try and rub her breast. Only one, and only do it for a second. Then go back to steps 3 and 4 for a while. Soon enough, she'll be dying for you to take her bra off, so slip it off and have fun.
This method, while still being experimental for me, should save you the trouble of her blatantly shutting you down like that.
Good luck.