What made her want to go through my phone?

seriusly i have never cheateed on her, never hung out wiht girls who are really good frineds of mine unless i'm with her, i caught her twice trying to get into my phonefirst time was few monthes back she knew the pword coz its always my bday its my phone so i dont go crazy and again yesterday, but i have a new phone wiht the fingerprint reader so she kept asking me why i lock it and to unlock it coz she wants to play with it which is complete BS she has the captivate same as the atrix an android phoe nothign is different. why the change all sudden been going out for 4yrs now, the only thing she found out was i was a message from an old friend on FB (on my phone) she just said "remember in ____'s cl you wrote me a poem that was so cute" it was a english cl in 10th grade now we are 22 and we had to write a poem and i wrote it to this girl that i had a crush on. she kinda made a big deal of it but it slowly went down that was ages ago