Girls, What does it mean when a girl smiles when you go to help her?

This girl at my work who has been acting a bit quieter with me recently. I've known her for a few years. Our conversation was effortless and easy to start with, we just clicked so well and our eye contact was really strong. Now she seems to be acting a bit awkward around me for no apparent reason. She did smile a bit the other day when I walked over to help her cause she knocked something over. When then started chatting. She seems to still laugh at things I say. But initiating conversation everytime I see her seems a bit awkward, it's making me nervous and I think she is too. But we've spoken heaps and known each other for a few years. So what gives?
Girls, What does it mean when a girl smiles when you go to help her?
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