What bath products do you use? Do you have excema or any other existing skin condition?
Do you come into contact with any other potential irritants like swimming or being around animals or certain plants or chemicals?
Have you tried the gentle unperfumed ones, like the Sanex and Carex antibacterial ones or hypo-allergenic products? Make sure it has a good neutral Ph level. Try and avoid those containing sodium lauryl sulfate.
Do you live in a hard water area and have you moved recently so change is water?. If hard water you could try a water softener or, generally, a chlorine filter.
You could also test the Ph of your water.
Could be from the heat of the shower (like a prickly heat reaction), have you tried at different temperatures?
Have you got dry skin which could make it worse? If so then try and improve the dry skin as well and use lotion (again unperfumed) to rehydrate your skin after showing. Try and keep showers short and not too hot.
Dry your legs and put clothing on as soon as possible after you come out of the shower.
Make sure you drink lots of water as well, as pure as possible.
You could also try an antihistamine to see if that helps.
Give things a few days to work though before you write them off.