Since you have a pretty well-defined point at which you started seeing the increase, go back to before the increase in the reports (hurry .. you only have 90 days!) and run an "Account Activity Detail" report. Then, go to just after the increase began, and again to a very recent date and run the same report for those periods. Download the reports to your system and begin to look for specific terms that may account for the increase.
Once you have identified which terms are causing your increase (and it's probably some specific terms, not overall activity), look in your server's log files for those same periods or use whatever log analysis tools you are using to find entries that match searches for those terms in the referrer. If you are using a "tracking code" to identify your PPC referrers as suggested by the engines, your job should be as easy as filtering the logs for those codes, which should match up with what you are seeing in your reports.
If there is any obvious fraudulent activity, you should see patterns that will confirm your suspicions. This is the data you will need in order to convince Yahoo/Overture to consider giving you a refund, so keep it nice and tidy. Their first line of defense against rebates is "we already credited you for those clicks with our excellent click fraud detection program" ... ahem ...
Of course, fraudsters are a bit more sophisticated than they used to be, so patterns may not emerge cleanly. You may need to look at IP addresses within range of each other, time and geographic position relationships, and other stuff like that to find a pattern.
Good luck to you. ;)