Teething does make nappy rash a lot more likely as their wee contains a lot more ammonia for some reason. We have just had this with Noah, and the problem is you cant keep the area dry and clean in a nappy! Ihave had him on hydrocortazone cream from GP, but in the end Ifound what got rid of it was giving him 2 days of nappy free time, which wasnt as bad as I thought but then a lot of my floors are laminate! He just got a bit upset at doing a poo on the floor, first time he had seen himself do it! but it gave the sores a chance to properly heal, much like having a couple of days cleaning up after a puppy!
Do you use cloth or disposables? There is more chance of a reaction in disposables due to chemical reactions. I stuck Noah in cotton terry nappys with lanolised wool and it let his skin breath and stay cool over night and his skin felt slky soft in the morning.