what is this on my dog?! *warning: gross*

I’m posting this on here because I have no idea what else to do. It’s 12:18 AM on a Sunday morning. I found this on my dog (she’s an 11 year on west highland terrier). I was petting her and I felt something very crusty and she was growling. I tried to move her fur around so I could get a better look and she was growling very aggressively (which she does when she’s in pain.) I’m really scared because I can’t find anything like this on google, my parents aren’t answering my text messages, and the vet is closed until Monday.

Has anyone ever seen this on their dog before? I’m extremely worried about my dog. She’s my baby and I hate seeing her in pain 😔 anything helps!