Deskrider, some of the girls in our group used to get the same when the temperatures went up, it is caused by bacteria that tribes on hot warm conditions. The secret is to avoid it instead of treating it. Use a chamois cream that last during most of the ride and keeps bacteria off. The girls and I use Endur-It, this cream lasts several hours and keeps the bacteria off, but I am sure there are other chamois creams that will also work for you, just have to try them.