I would not be very concerned if the 2 year old just started saying no touch during diaper changes as it could just simply mean he hates having his diaper changed or having his bottom wiped and cleaned up.
My own 2 year old son said no touch as well when I would change his diaper and he didn't go to daycare.
It turned out that he hated the cold wipes wiping his bottom and didn't like me touching him down there with the wipes.
Also some 2 year olds may become embarrassed about you changing their diaper and not want you to clean them up down there.
However I would still be aware of anything that may be potentially happening as anything can happen.
But it does not always mean that something bad has happened to your 2 year old at daycare during diaper changes.
Maybe his penis his sore as well and check for any small rashes on his penis that could have resulted from being in a diaper too long.