Thank you for sharing your thoughts and feelings. I will try my best to share my opinion, but I would like to make sure I understand your situation. Feel free to reply and correct me if I misinterpreted your question.
The most obvious concern that I understood is that you feel lost and confused because it is the first time you fall in love with a lesbian. Love is a very powerful feeling, and it is not always easy to follow your heart. Sometimes you feel at the top of the world because that person smiled at you, and sometimes you hurt when that person hurts. Love is an adventure. First step: listen to your heart. What does it tell you? For example, does it tell you that you should ask her out on a date? Or does it tell you that you are ready to tell her about your feelings? Or maybe it tells you that you should get to know her better first?
Personally, I find that I am happiest when I go where my heart leads me. And you are right, I follow my heart no matter the gender of the person I like. But it is trickier when I am uncertain of his or her sexual orientation.
If I understand well, you are uncertain your officemate's sexual orientation, because she denies that she is attracted to women. You do not specify the nature of your relationship with her: are you comfortable enough to ask her directly? You could ask it straightforwardly, but in a casual/fun way. For example, how about asking her Which celebrities do you find most attractive? or Who would be your number one fantasy date?. Another alternative would be to tell her more about yourself, your romantic history and hopes. For example, maybe after you see a movie with her, you could say I would love to feel the same way about a girl as the actor did for that actress or I cannot remember the last time I felt so strongly for a woman. The most important thing is to build comfort and trust. If she sees that you open up, maybe she will too.
Another concern that I understood from your question is that you are unsure how to interpret the way she interacts with you. Did you consider the possibility that she might be unsure of how she feels? It could explain the mixed signals that she sends you. In this type of situation, the fastest way to find the answer is to be direct Is it me, or I feel like you are flirting with me? I like it! You could say it in a playful and relaxed way, so that she is comfortable answering the truth. Alternatively, you could also take your time and see if your interactions correspond more with friendship, or with flirtation and romance. The advantage with taking your time is that you allow her to get to know you better, and as you both get more comfortable with each other, new feelings might develop.
My main message is that you should follow your heart. It will guide you. And even if the outcome is not the one you would have hoped for, it will make you stronger to know that you listened to your feelings. If she is a true friend, she will remain your friend, no matter the outcome. It is difficult to explain love. It just happens. And you should take pride in such beautiful feelings and allow yourself to feel them.
Being in love helps to get to know yourself better, even if it involves sadness and confusion along the way. And in my opinion, we feel freer when we feel strength, pride and completeness within ourselves. This allows us to give out to the world what it has given us, in the forms of quality friendships and relationships.
Take care, take heart,