This girl keeps sighing?

Everytime me and her finish having a conversation and depart from each other and I would hear a big sigh coming from her part maybe if it was once I wouldn't mind but she's done this 3 times now and the strange thing is everytime we engage in conversation she seems really lively and bubbly but when we end the convo and I begin walking off she gives me this huge sigh.

What do these sighs mean?
This girl keeps sighing?
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