All file-sharing systems work on the basis that, while you're downloading files from someone else's PC, others can download shared files from your PC. So it's unsurprising that your PC identifies uploads taking place, since that means others are accessing your shared files.
Incidentally, it's the fact that you've got files available for upload that makes the file-sharing (without permission or payment) of copyright materials a criminal offence, as well as a civil one. (It's only a civil offence to download files, other than on a large scale or for commercial gain, but making files available for upload counts as 'distribution', which is a criminal offence).
Chris