Hello, I've Had A Bump/lump On My Mastoid Bone Behind

Hello, I've had a bump/lump on my mastoid bone behind my right ear. It does not hurt to touch it or press on it, it feels semi hard like a bone? But I do not have the same thing on my right ear. You can see the bump to the naked eye. I do not have any other symptoms. It is as big as the nail on my index finger. If you might know what it is, I would love to know. Thank you!


Also, I am female 21 years old. No other previous health conditions. I do take Spirolactone for my acne.