We boys (& men) are hardwired to "mate"---it's an atavistic response, especially if you were attracted to her previously and now she has told you of her "feelings" for you.
Fortunately, we don't have to act on every impulse we have; and my suggestion to you is to not pursue her in any way.
Concentrate on the moment on your relationship with your current girlfriend---there are far too many potential negatives involved in attempting to pursue this second girl.
(And look up atavistic if you need to---it's a cool word.)