It sounds like a small muscle spasm. They're usually harmless but if you're worried, call the vet.
Bella wasn’t born with it, I only noticed it a couple of weeks ago and I’ve found pictures that show no debt or mark.
It’s probably just a birthmark, if you can fit your thumb in the indent it is just a birthmark. Some horses are just born with it, they say that all the horses that have this indent are descendants of this beautiful horse a king owned, and that all of his foals would be born with this. Lol it’s just a legend but it’s nothing to worry about.