Moms need help fast!

My 4 month old baby girl got shots last Thursday. Well me and my fiancé went out of town for a weekend and had my MIL watch her. When we get back Thursday and she brings her to us. I was changing her diaper and saw this. It's really hard, when I barely touch it, she starts to cry out. I'm upset cause my MIL failed to tell me this even when I called her to check up on my daughter.