strange spot on ear

A couple of days ago I noticed a small brown spot on the outside of Crumbs' left ear. I thought it was just dirt or something, so I wasn't worried then. However, I was petting her just now and noticed that the spot is darker now, and raised (it was flat before). Does anyone know what this might be? Does it warrant a trip to the vet? She isn't bothered by it at all.

I don't know if it's the same thing but Sophia has one on her ear too and Bella also had one, the vet told me it was an age spot...but if you have any doubts at all I would see the Vet.