When I touch the front of my lower leg, it feels like a pea sized bit of cartilage is loose under the skin. It doesn't hurt, but is something wrong?

Cyst or lipoma. It's unlikely to be cartilage, but it could easily be an epidermal cyst or a lipoma. Both are benign, both are easily removed under local anesthesia as an office procedure. But since there are other, less common things that it could be, it would be wise to allow a dermatologist to have a look at it in person.