vaginapagina | Bicycle-appropriate panties required?

I have recently started riding my bike around town again, and I have discovered something that I don't like at all. It seems that if I ride for more than 20 minutes wearing American Apparel boy-cut panties or a cloth pad, I get blisters on my outer labia! I'm pretty certain it's not herpes because these lesions occur pretty much exclusively while I am on a bicycle, and I've been tested in the past year for herpes and I am without antibodies for types 1 and 2, and I am in a monogamous relationship of a year and a half.

What I want to know is: does anyone else ever have bicycle-related vulva blisters? What seems to cause them more often, and what fixed them? My roommate suggests sport panties, but I want to see if there's a quicker fix at home. I just don't wear the offending panties on days that I bike (easy enough), but for IUD-related reasons I'm using cloth pads a lot for the next three months before I switch back to a cup.