I just returned from vacation... and see this thread already has good info on it from Koala.

I have a couple of other comments too...

1) soap residue down "there" can cause lots of irritation as well.
Women who shower and the soap suds, shampoo etc get down there and may not be rinsed off well, can set up lots of issues due to the chemical irritants. When washing down "there" use as little soap as possible. Some women find a drop of baby shampoo is the most gentle instead of soaps. The body protects those membranes and washing that protection off with harsh detergents/shampoos leaves you vulnerable to pathogens like yeast or other bacteria.

2) using bath salts, if you soak in a tub also can cause irritation and in some women lead to UTIs too.

3) and I agree with Koala about concentrated ACIDIC urine burning. If you have no frequency issues, or pain in the lower abdomen -- acidic urine may be a cause. Diluting urine with more water intake helps, or cautious use of bicarb or other alkalinizing salts may help.

4) there has been some concern that thong type underwear, may promote transfer of bacteria from the a n a l area to the bladder area. So using cotton underwear is a good idea instead.