Why is she acting like this to me now?

A few months ago this girl I know at work, she works in the separate office downstairs, really threw herself at me in a totally OTT way. For about a week or 2 she kept coming up to my department and really blatantly giving me the eye, wearing a short skirt to work, smiling, making stupid excuses to ask me things. I am a shy boy and I was kind of in shock and I was also dating someone else at the time so I basically ignored her and I did think she might be a bit tarty also which is kind of off putting. Anyway she stopped acting like this and hardly ever comes anywhere near me now. My date dumped me flat and I'm desparate and I suddenly fancy this girl, I wrote her an email last week asking how she was doing and asking her for a date and compliments all that but she ignored it and today I went downstairs she totally scowled at me like I was a total nasty loser. Why has she decided to act in such a cruel way. Why did she give me so little time.