well you know washing powder makes us women itch so try not sleeping with no underware on an use dial or safe guard soap
It sounds like you may have a yeast infection. Do you have any discharge? You can try an over the counter treatment like Vagisil cream for the itching but I would see your doctor for a prescription treatment. If you do try over the counter first, stay away from from the 1 night treatmen like Monistat1 because it will burn your irritation.
See your doctor. Plain and simple. Do you have any discharge? Does any of the itching happen more or less after sex? Is it the inside or outside...or both? It could be anything. The only way to know, is by seeing a doctor.