Two issues here: 1) the medical problem, and 2) how to avoid in the future. Good advice on seeing a doctor, for issue #1.

For issue #2: Mike, I think you told us you swapped drop bars for the original flat handlebars. This has probably changed your contact points on the saddle, and that in turn is probably causing the sores. Any time you change or move the saddle, the stem reach, etc. it can take a while to work out the bugs (so to speak...) Even accidentally twisting the saddle 1/4" has caused me problems.

So four things:
-- As soon as you make a change, pay attention so you can adjust other things back to normal. Maybe your saddle needs to be moved forward/back, lowered/raised, etc.
-- No underwear as cotton seams cause problems (you're doing that)
-- Lotions and creams, used properly.
-- Good hygiene during the ride, every night.

Good luck for next weekend!

-- Mark