Good evening girls, could you please tell me if anyone has had somethi…

Good evening girls, could you please tell me if anyone has had something like this? It's not an burn or a bruise, it wasn't there in the morning, it was there all day almost in a stroller because I ran around the city for errands, and by evening I said it hurt, lifted my t-shirt and then such a sore spot appeared. Like a blister popped, only appointment is on Tuesday online, they prescribed Tridox ointment ?I'm in despair, how quickly could it have appeared, the t-shirt was loose on her, I don't know, has anyone had a similar situation, please tell me ?photos were taken in the morning and evening