I need a lot of help and an interpretation of the situation with my crush?

About 2 years ago there was a girl in my college class who had a HUGE crush on me; she flirted with me aggressively: she would stare at me for minutes at a time and never look away every time I made eye contact with her, she would constantly smile at me, and she even eye f***ed me once. She consistently flirted in this manner for the entire semester. Despite this, I never talked to her or said a single word to her (and I'm not sure if I ever flirted back) because I was in a 1-year committed relationship at the time, which this girl was unaware of. However, after I broke up this relationship after the semester was over, I began to develop a crush on her too. Fast forward to the present, I see her again on campus, she notices me, she begins to blush and display signs of attraction towards me, this leads me to believe that there is still some chance between us, later during the day I approach her on her way to class, she receives me warmly but says she is in a hurry to take a quiz, I ask her out, and then after around 4 seconds of thinking, she gently rejects me (she sounded sort of sad when she did). Recently, I’ve seen her staring at me from the corner of her eyes with a sad/happy look on her face as she passes by me and then looks down; other times, I think she tries to avoid and ignore me.

Can anybody give me their opinion on this situation? Could she have COMPLETELY forgotten about me despite the huge crush she had on me? Might she regret rejecting me? What should I do? Has she completely moved on? I don’t think she has a boyfriend because she would've told me, instead of waiting so long to respond and just saying “no”. Can she possibly still have some feelings for me? Should I apologize and explain to her why I was never able to make a move on her? My theory is that she still finds me attractive, but is upset because I ignored her the whole semester.
I need a lot of help and an interpretation of the situation with my crush?
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