Thanks a lot for the tips everyone (especially those from tipster).
I removed the torrents in question and rebooted, and started downloading one new torrent to see if the problem would remain. For a second or two the torrent's own dowload speed itself went down to 0.0 kbp/s but the program continued to have a global download speed of about 13 bp/s, just like it did when I was seeding, which led me to believe that this must be normal for the program.
The only strange thing is that I'm pretty sure I remember seeding a bunch of other torrents while not downloading anything before and the global download speed stayed at 0.0 constistently. The only difference was that those torrents were normal torrents, not torrents I downloaded from the private tracker I recently joined. Perhaps the private tracker requires more temporary files during seeding, because it uses a special mod for torrents that have no peers but many seeders (which included me at the time).
Perhaps I will investigate the issue further later with aid from your other tips, but the good news is that after I removed the original problematic torrents from seeding (without deleting their actual downloaded contents), my total free memory went up from about 6 GB to over 10 GB, leading me to beleive that whatever data had been used up was from temporary files that Azureus requires, explaining the continued download speed during seeding only. Still, that's a lot of space for temporary files, even though I deleted a few in the mean time.
So apparently the problem's solved for now. Thanks again to all who helped, it's much appreciated.