Why would a guy suddenly start ignoring/being rude to a girl?

I am crushing hard on my coworker and I thought he was crushing too. He was really nice to me, did favors for me, was polite, awkward and quiet around me not everybody else, he stared at me all the time and when I looked, he looked away quickly. We used to talk and be friendly with each other, but he hardly said anything when we were alone together. He was so quiet almost like a different person.

Now suddenly he acts like I don't exist. I didn't do anything. He talks to everybody and is friendly and then just walks passed me without looking or saying anything. I said hi to him once a few days ago and he just nods and smiles a bit then walks away. Didn't say anything to me for the whole day and has ignored me since. He was talking to me and being nice the day before. I'm hurt and feel guilty of something. He is really going out of his way to ignore and avoid me. I can tell. I think he hates me sometimes. Should I break the ice and ask what is going on and if I did something or wait till he comes to me? Also, why is he doing this? I know he still doesn't have a girlfriend but even if he did, why can't get treat me like he does the other workers?

p. s. im really shy and may come off as disinterested
Why would a guy suddenly start ignoring/being rude to a girl?
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