My (29M) co-worker (23F) has suddenly stopped talking to me. What should I do?

I have been working at this company for a while and there’s this girl who joined after me. We had been getting along well these past months and chit-chatted. But suddenly 2 weeks ago she stopped talking to me. I wish her good morning and she wished me back, but without looking at me and in a monotone voice. She’s also avoiding looking at me whenever she passed by. I sat next to her a few times when I was talking to another co-worker and I looked at her still she ignored me, even though I’m sure she knew I was sitting there and she wasn’t too busy at that time. I asked her one time “are you busy” she replied “yes” with a stoic face and didn’t even look at me. She was just sending one short mail and usually she turns her head while talking and speaks properly, but doesn’t do that now. I sat next to her few days ago and I asked her if she’s angry on me. She didn’t look away from computer again and said “no” with stoic face this time too. I then asked her why she’s not talking to me she said “I’m busy” and didn’t even look at me. I don’t know what I did but I can tell she’s angry with me as she’s her usual self with others and she laughs and talks with them. I want to confront her directly but I don’t want to provoke her temper. What should I do?

TL;DR – co-worker suddenly stopped talking to me and denied being angry though there are indications that she is.