It would be very unusual for you to only get itching and burning without sores if it were herpes.
Do you see any redness, rash, or bumps? You could have a fungal infection - think yeast, jock itch, things like that.
You could also be having an allergic reaction to something. Have you recently changed any products you use - laundry detergent, soap or body wash, toilet paper or wipes? Even if you use the same brand, perhaps you've changed scents, or the product changed formulas, usually "new and improved". Started wearing a different kind of underwear?
You had protected everything. The chance of you getting herpes is very, very low, around 1 in 10,000. (Your chance of getting struck by lightning this year is about 1 in 3.000.) Syphilis wouldn't cause an itch like this - you'd only get a painless sore. HPV probably wouldn't appear this soon, and if you got symptoms from it - many don't - it would be bumps, not itching.
So watch yourself for any redness, bumps, rashes, etc. If you see any, get checked by a doctor. If the itch doesn't subside in a couple of days, see your doctor. (When I say watch yourself, I mean check once or twice a day, not every hour or anything - that will just serve to irritate already irritated skin.)
I'd be really surprised if this is an STD.