NEED HELP "Taking a Break?"

Well my girlfriend who I have been dating a long time and I love with all my heart just told me last night she wants to take a break... the same girl that has repeatedly declared her love for me. I feel we have had a VERY strong relationship and things were all good. So whe comes over last night and then we are kissing and she stops all of a sudden... I ask if something is wrong and she says she doesn't want to do anything so I say sure whatever you want but ask if she could explain the problem... she says not to worry about it. Of course I do and not much is said the rest of the night. Well... abotu 15 minutes later I get a call and she says she is sorry for leaving on a bad not and that her horomones have been acting up and making her emotional and crying before bedtime so she wants to take a break and that is all she says... i was too stunned to say anything. I don't know how to take it... does it sound like this is sincere and could that actually be a valid reason for wanting to break things off for a while... she didn't say it to my face and didn't give me any amount of time or tell me she would call me or anything, just totally left me hanging. Please anyone tell me what you think, I am in total confusion and have no idea what I need to do. I sent her a test message earlier telling her how much I care and I am here for her and asking if she is sure she wants me as a B/F and that her reason is true... HELP!

I know, sometimes it's hard to believe you've been rejected, but *why* would what she said be insincere?