On the other side of the scale... cooling things down also have a good pain relieving effect, especially on muscular inflammation which is common both with whiplash, rheumatism and fibromyalgia.
Ice reduces swelling and increases blood circulation, while providing temporary pain relief by numbing the local tissue.
I have found that ice packs or simply (and more cheaply) just a bag of frozen peas can work wonders on my whiplash pain and prevent migraines if used early enough. Sometimes if my back is acting up at the same time, I need to sit with a heating pack or hot water bottle on my back and an ice pack on my neck simultaneously. That does make me look rather silly, but it really works, so hey, who cares? Both heat and cold can trick the sensory signal system and break a pain-spasm cycle, providing a simple and effective natural pain cure for many types of muscular pain.