It happens in most relationships. Someone new comes along and shakes everything up. Reassure her that your feelings for her haven't changed and try to make time to spend with her. Don't panic, this is so normal.
Bratty is totally wrong, you may actually not be spending enough time with her, you have to find a healthy mix of spending time with your boyfriend and spending time with her, dont just drop her because she feels like you are spending every waking minute with your boyfriend instead of her. Why dont you just reassure her, and schedule to hang out with her sometime soon. Another answer is to find her a boyfriend and double date.