s hard to say what the cause is without seeing them but a few things could be chaffing, which is the rubbing of your inner thighs, exzema, an allergy/intolerance or infection.
Have you tried
aloe vera gel? Make sure it is pure.
Aloe vera is a miracle for all sorts of skin ailments including rashes, burns and acne. Apply this liberally for a day or two and you should notice improvement. Don't scratch or rub as this only irritates the skin more.