Girls, Why would a girl smile and quickly look away?

Anonymous
I started noticed a girl in the gym some weeks back. I though she was cute so I started checking her out. At first she didn’t really look back at me. However , would sometimes sneak glances.

My friend personal trained her, he is attracted to her and try’s to use his job as a means to flirt with girls often. He is a slightly older man. When he was personally training her she kept looking at me and sneaking glances. I feel it made him jealous , also he kept looking at me when he was with her and he would not speak to me for days so was.
I saw her in the local store two days later , she looked at me and smiled.

When she saw me about to walk passed her she would smile looking at the ground but not at me. Also she passed on the stairs , she smiled looking at there ground and did not make eye contact again?

I noticed she looks at me when I’m not looking , or if I walk off. Why does she look at me and smile in store , but then in the gym will smile but avoid eye contact.

Should I add her on a social media?
Girls, Why would a girl smile and quickly look away?
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