Not A Poet
Nearly every line of love poetry that can be imagined has already been written at least several dozen times in one form or another. So it is highly unlikely that you will write a line that is not similar to something already done. But since there are so many lines possible, if your poem is more than just a couple of lines, it is certainly understandable that she questions the similarity.
Maybe you did copy someone's work and try to present it as your own. That's pretty tricky with the internet out there today. Maybe you read and were impressed with the other work but forgot it then inadvertently reproduced it while thinking it was your own.
In any case, you have a big problem here. I hope you can find a way to work it out and somehow make things right. But it aint gonna be easy.