Co-Worker flirting , what to do...

So I have this friend that I met through work. She calls me to work certain gigs, but we’ve also become good friends over the last few years. We will sometimes dm over the course of several days, weeks, just continued conversation and her asking me questions. To me this feels odd because I don’t even have continued conversation with my best friends over that long(we are both mid 20s). But also if I cross a line of directly flirting or asking if she’s into me I risk ruining a good work connection.