1. Is this your normal sexual orientation ?
2. Do you have any clues about the other lady's - does she (or has she had) have a partner of either sex ?
3. If you answered 'yes' to 1. - does she know - and would it worry you if she did ?
4. Is this just a physical thing or do you think that it could go deeper ?

One way is to try and find a similiar situation to the one in which she kissed you and kiss her back.
If she does have a current partner, you could be in a messy situation though - perhaps even more so if the partner's male.