If some one is feeling not the same as you then it shouldn't keep going.Your friend might say its ok,only because she doesn't know how to say NO! Even what seems to be harmless can at times not be.So its best to just stop what your doing before it gets out of hand and then you regret it.Just find friends that like you back and leave the girl alone. :-)
Of course it's ok to flirt with a girl. They won't tell you, but they like it too. They need your attention like you need their attention. You can try to use different tricks from lovedignity.com/ , this will make everything even better. It is very easy to flirt, and it is normal!