This will sound weird, but try "trimming the forest" back a little bit in the offending area. Sometimes I have similar problems if I let the hairs get out of control. This totally fixes it.

Worth a shot....and it's free.

Also, your saddle hasn't moved side-to-side any, has it? Maybe been bumped and moved just a tad to one side?

Also, you're washing your shorts every single ride, right?