Hi Sarah,
What you are likely describing is the mastoid process, a protuberance of the posterior temporal bone, and a site for muscle attachments. You should be able to feel such roughly equally on both sides. Though it is possible in some cases for the mastoid bone to become infected, if it isn't tender to touch I likely wouldn't worry about anything. As well, an infection can cause swelling in the lymph nodes behind the ear, though this is unlikely to feel as hard to the touch as you describe.
Though if you are concerned, it wouldn't hurt to have a doctor take a look at it.
Hope this is of some help or reassurance. Take care.
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