Yes, try using only water. When you do use something down there, I'd suggest using a feminine wash that is odorless without perfumes/dyes and is hypoallergenic.
Could also be the onset of a yeast infection so keep a look out for it getting worse and any odor or discharge. Might not be a bad idea to apply some Monistat cream or anti-fungal cream just in case.
I hope you feel better soon. Nothing worse than discomfort "down south."
If this is the same thing I had experienced in the beginning of my pregnancy, it may be a yeast infection. I know you said its only on the outside, but mine was the same way. Call your doctors office and ask his nurse if it would be ok to just go buy an over the counter medication. I am prone to yeast infections and my doctor told me that I could self treat them as long as I was not buying the kind the one day treatments. If you are going to self treat for a yeast infection during pregnancy, he told me to buy one of the ones that you continue for multiple days, the seven day one if possible. This way, your exposing the baby to smaller amounts of medicine over a longer period of time, rather than alot of medicine at one time. If I remember correctly, you are only a few weeks behind me in your pregnancy, so this should be okay for you. Since your experiencing the irritaiton on the outside, make sure that if you are able to go buy some medicine, that you buy one that has the external cream that comes with it.
Im not doctor so of course you may not have a yeast infection. You may just be rubbed raw for some reason, or be allergic to something. Have you used a new panty liner or anything? Im sure if you just wait it out you will be fine but I would call your doctors nurse and see what she thinks.