Illness/Sickness: Black spot on head?

[Alienz]

I noticed this black spot on my axis head and at first thought it was just dirt from the pellets I feed her, but the spot didn’t go away and got darker! It might be a bruise from her swimming into something's too hard as shes still eating fine and I dont see anything else wrong but I just want to be sure its not something i should worry about or if I should do anything for her

Any help if appreciated