I feel your pain! This has happened to me before and is most upsetting, however try to view it in a positive way and as a learning experience. Imitation is the sincerest form of flattery, and obviously this colleague thought your idea was brilliant. To keep the peace in the office, I advise that you refrain from confronting her, and instead, be the bigger person and let it slide. The next time you have a genius light-bulb moment, go straight to your supervisor to share your idea (or present it during a full team meeting) so you can get the praise and credit you deserve. Good luck!