Why would this girl act quiet with me given we know each other and are on good terms?

Nothing bad or anything has happened between us. But for some reason she is acting differently towards me. She just seems quieter with me. For example, today she just walked past me and good morning to another person. She normally says it to me but I noticed lately she seems strange with me. What the hell is going on?

Why would this girl act quiet with me given we know each other and are on good terms?
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