Questions: Girl's Behavior

I’m confused, she started a while ago she works sending the orders to the industrial printers and cutters that I operate. We barely spoke for like two months but then one day I broke the ice just with work talk asking for files to be sent over or the work note so I could complete the order. I look fairly grumpy at work all the time I’m polite and helpful if you interact with me but my true emotions always show on my face willing or not. Anyway one evening I walked up with the last note for her to fulfil just as she was about to leave and she said “ugh, thanks..” that made me laugh and I just said “your welcome” and since then whenever I go to hand a note in and she locks eyes with me I can’t help but smile and she always smiles back now. She got her revenge recently as she printed a stack of papers that had to be done that office had sent down five minutes before the end of the day, same smile from her and a giggle as I called her out on it. Then today she was talking to Michael my other colleague near the end of the day and I was lifting some heavy laminate material for the laminator, Michael broke conversation to ask if I wanted a hand and I took a playful jab at them both and said “nah you two keep prattling” and she retorted by saying “I can come prattle with you if you’d like?” So I said “nah some of us are here to work” in a jokey way and yeah just general playful banter back and forth for a few more minutes. Come to think of it we don’t even know each other’s name we’ve just developed this jokey sense of playfully annoying each other. I don't know I feel like I’m reading into things too much, regardless I’ll just keep it causal and let whatever this is run it’s course.