Your dentist is correct. What you have are mandibular tori. You can have them on both sides, or just one side. You can also get one of the roof of your mouth called a palatal torus. Torus is singular.

They are VERY common. I see them probably everyday. You don't need to be concerned about it unless it becomes painful OR you need a denture in the future as they sometimes need to be removed to make room for a denture which can't be pleasant (BRUSH AND FLOSS!!)

It's also pretty normal for them to VERY gradually increase in size over the years.

Gotta love the internet and how any little thing you try to research makes it seem like you've got some rare deadly disease!