You say that she said that she would prefer that you stay friends and never mentioned her feelings about you, and that you continued to aggressively push the subject to know where the friendship was going, but she wasn't budging...And she says she prefers being single...
You work with her occasionally, you say? Having a relationship in the workplace is difficult. She may be reluctant to start something and then have it go sour. Is she someone you can call on the phone and talk to anytime?
You sound like a sweet person. You might start by asking her if she feels that what happened between you was a mistake-of course at the risk of getting hurt, but you will most likely get a straight answer. If she says no, would you be able to take the initiative and tell her how you feel? If she says yes, can you handle hearing why? Did she ask you for space??
We all know deep down that sleeping together is meant for AFTER you are in a committed relationship. It's hard to do though! But there's a reason for it...you KNOW where you stand with the other person. I'm really tired and don't feel that I've helped much, but I do wish you luck in finding the truth so that you can either pursue a relationship with her or start the dreaded process of moving on...
dizi