I get a brief warm sensation down the back of my calves at random times during day. no pain. chiro said nerve related? could it be something serious?

Warm sensations: These warm sensations could be due to irritability of superficial nerves or muscles in the calves caused by activities during the day, improper walking, prolonged standing on hard surfaces, or certain footwear. They may be variants of muscle cramps or tingling sensations seen in deficiencies of calcium, magnesium, sodium or potassium. If you're unable to pinpoint the cause, see your physician.

Possible: Why are you relying on the opinion of a non-physician? A chiropractor is a therapist, similar to an average physical therapist, and has no extensive education like an MD or DO physician. Your situation needs to be evaluated by a knowledgable practitioner.