If she is not too fussy about such things then tell her regarding your fling emphasizing that it was before you even knew her. However, if she is hard to please in such matters then never talk about this. If it was just fling, then you should not feel guilty and there is nothing wrong in hiding such insignificant facts. Otherwise, one good relation will be tinted with negative feelings. You may tell your friends about your confusion and not to tell anything to her in future, if you really trust them. Or otherwise take a chance.