something similar happened to my DS in the last few weeks also. he is 10 months and switched teachers in his nursery (from infants to active babies). i am sure that he isn't being changed as frequently. and for a few days in a row, he has come home with poop in his diaper. his old teacher was very conscientious about changing him before i picked him up, but the new teacher isn't as good about it, and because i had gotten used to him always being clean when i picked him up, i never check him when we get home, so he would sit through the drive home in a dirty diaper. he got the rash with open sores and we treated with Rx cream and then thick desitin and corn starch powder. it's almost gone now.
we told his teacher to change him more frequently, and i always check him when i pick him up now.