Thousands of really long URLs are being generated for my site

Have you tried:

– deactivating all plugins to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s).

– switching to the default theme to rule out any theme-specific problems.

– resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems.

– resetting your custom permalinks back to the default setting via Settings -> Permalinks. If this works, then review Using_Permalinks before setting a custom permalink structure back up again.