i can understand your concerns ,
there are two possibilities in such age,
1:Yeast infection:very common in babies,you should Lamisil
ointment twice a day for 5-7 days. If you feel there is inflammation aswel the you can try Clotrim-HC cream.
2:it also be an irritant contact dermatitis
due to stools and urine or may be due to a diaper.In such case i would recommend application of white petroleum jelly or Vaseline after every wash and in the night. Also to reduce the inflammation kindly use 1% Hydrocortisone cream
once a day for 5 days.
i hope it will get better with the treatment.