Since you were not romantically involved in an exclusive relationship, the fact that she slept with someone in the past should have no bearing on your feelings for her or on your relationship.
The fact that you two started out as friends is actually irrelevant to this discussion. We all arrive at relationships with pasts. Those pasts should stay in the past.
Take the time to read this very long thread for some information that I think you will find helpful to clarifying your thinking.
Good luck with the new relationship.