Cycling shorts are made to be worn with nothing under them. The seams in your brief's will certainly cause you some irritation, though I don't expect it's the problem you're currently having. Before investing in a new saddle, I would give it a couple of weeks and certainly more mileage than 25 miles to see if you adapt to your current saddle. I was told this by many people, both at the bike shop and seasoned riders. I'm still new to road bikes myself though, so this is the best advice I could personally give you.

Good luck!