<human>: Could you distill the essence of the text into 1-3 sentences?

I live with my girlfriend of 2 years this month we love each other very much we live in an apartment so we share a communal laundry room. I did some laundry yesterday and my girlfriend and get up sorting it out and folding it, in the process she found a pair of women's underwear that are not hers.  This looks horrible and I'm completely devastated.   At first she was freaked out but after a bit she came to realize this is a very possible occurrence, myself however did not handle it well and just felt (still feel) very broken down, ashamed to say I cried through most of the night.  I just don't want to lose the woman I want to spend the rest of my life for something I never did.  

She wants to keep this totally between us which I believe is the best plan going forward, however I'm worried that she may not actually be able to keep this to her self and when it does get out with her friends / family, its not going to be pretty, and I worry would be the beginning of the end.  

I just want to know why?  I feel so helpless and all evidence points to me?  Is this karma, for something I've done in the past? What can I do going forward to stay positive, keep our relationship strong/alive?  Prove my innocence if possible?  I just want to back to the way things we're please give me peace.
<bot>: I did some laundry yesterday and my girlfriend sorted/folded it and found a pair of women's underwear that are not hers, communal laundry in our building, I have nothing to do with these and its killing me inside.