It is hard to see but it looks like a tear or puncture between the heat pit and 'lip' I actually had a similar issue with one of mine, she hit a rat claw and punctured at this point. I went to the vet and as expected I was told to use a silver sulfadiazine cream. It is likely this will work although make sure there is not sign of infection and you keep it clean (switch to paper for the a while) I dabed a little bit every day for 3 weeks and it cleared up completely. If there is any change you would need to go to a vet. It might be worth going to a vet anyway.