Advice on friendship?

So I started getting drinks with a co worker. I asked her first and she agreed as long as I remembered she was married. I respected that and I have no intentions of crossing the line.
Just a few things have happened that I’ve taken note of like she has completely fill me in on the issues she has with husband and has admitted that she has no intentions of telling him that we hang out after work sometimes. I don't know just the vibe I get when we hangout occasionally feels different than two friends getting a drink.
Advice on friendship?
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