It's because you have a good thing going and you don't want to blow it, don't over think it or you will.
Watch what you're thinking, sooner than later you're going to live it.
Because you really, really, really like her. We (men) can be really weird that way. When we really like a girl and she looks at us we can go to pieces inside. We trip, fumble with door knobs, drop stuff, ...
If she is flirting and letting you know she is interested, pull yourself together for a minute, walk up to her, and say, "Hi. My name is ... and I would really like to get to know you."
Studies have shown that no matter how bad the pickup line or how nervous the guy is, if the girl is interested, she will engage you in conversation. If you don't do anything, you have already lost, so give it a shot. If she say no, at least you don't have to get butterflies anymore.
It's sin living within you, you may have been hurt in the past, a little bit of advice inner beauty comes from a calm and gentle spirit, don't fret, be yourself, speak what is in your heart and see where it goes from there.
Because you might tend to overthink stuff with her around..just go with the flow..be natural..be yourself
because you like her a lot... ?