Why would a woman look another woman up and down quickly?

There is this woman I work with and she is relatively nice to me in person but in the corridor when we walk past each other she will say hi to me and give me a smile but at the same time she will quickly give me the elevator eyes and look me up and down from head to toe, particularly focusing on my waist and hips. This makes me feel self conscious. I dress well and I like to keep in shape. My fashion sense maybe is a bit extreme for work, I like to wear bright colours and floral patterns. I’ve had this a few times from other women too.

Why would a woman give another woman an “up and down” look for no reason?

Why would a woman look another woman up and down quickly?
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