computer/credit card help-help!

hi! this is my first time on this forum. i will try my best not to look too dumb. here is my problem.
i recently made a trip to st.louis to visit my sister. while i was there i used her computer to place an online order for clothing. when i was done, i logged out of that particular website and closed the browser also. that was in late november. i was looking at my credit card statement online last night and saw a purchase i absolutely did not make. i called the credit card company this morning and they are doing an investigation. i called the retailer's number also and it turns out that they had my exact card number but (and here is the kicker) my sister placed the order online and had been a customer of their's for years. when i talked to my sister today, she said she placed the order the same way she always did and that her computer must have stored my number somehow.

don't get me wrong. i am not accusing my sister of anything. she knows even less than i do about computers. but i just can't understand how that can happen? my question is how can it? or can it at all?

thanks so much for any help. debi