diaper rash

I just started introducing milk to my 12 month old, formula + milk mix a couple of times a day. (she turned 1 on the 12th), since starting the milk she now has a diaper rash. she never had this rash until the milk started. it's not anywhere else just her private area... I keep her clean, I change her when she needs changed. is this normal? should I try maybe lactaid, could she be allergic? she was on Enfamil gentlease... I don't remember my others getting a rash when switching to milk