Cells automatically filled

Hi everyone!
I知 having a problem that I can稚 solve or find any solution to... I知 so puzzled...
I have two excel workbooks (A and B), each with several worksheets. The workbook A has all the past gathered values. The workbook B has a new layout, to use from now on.
So, I知 copying the data from each of the worksheets of workbook A to the correspondent worksheets of workbook B. The problem (mystery?) is that as I copy the values to the cells of one of the worksheets of workbook B, the other worksheets in this book are being automatically filled with values from workbook A...
Is there any way of turning this automatic option off?
I appreciate all the help,
Thank you so much!