<human>: Can you provide a brief overview of the text?

Me (18) and my gf (17) are currently freshman in college. We have been together for about 6 months now. We have been in small fights, but resolved them together. Everything was going great in our relationship until tonight. She used my laptop to go on facebook and when she was typing her email in the login bar, her dads email came up. (both emails begin with an "a") as if it was logged into at one time. She asked why it was there and I told her I had no idea how. She doesn't believe me and I feel completely insulted by it. I understand that I am not perfect and yes I have lied to her before, but I realized my actions and took responsibility. I know it may be hard for her to believe me. I told her I have no idea how her dads email address was saved in facebook on my computer. Her younger sister used my computer a week prior and I know she went on fb, so I thought maybe she logged into her dads. She claims she didn't and her sister (my gf) thinks I logged into her dads fb. My gf is very mad at me and doesn't want to speak to me until I admit that I logged on. I can't prove I didn't do it. She even went as far as breaking up with me over text meassage ( witch I find to be lame) and defriended me on fb as well.  So reddit, please help me. I don't know what to do. I don't know how to find out how the email got on fb. What do I do?
<bot>: My gf thinks I logged into her dads fb through my computer and I have no idea how the email login got saved onto it.