This is due to a lack of circulation. The blood isn’t pumping to your extremities so well, which in turn makes you cold.  Same is true in your bum, perhaps from sitting too long (on the computer, etc… especially on a hard surface).  Moving it – exercise – well of course help, though we need to clean up our blood, as circulation only partially from movement.  If the blood is clogged with mucus (this is most common for everyone as we tend to eat foods that produce on over production of mucus), yeast, and general toxins – then naturally the blood will stagnate – this is where things that make you feel crappy grow… and this is because oxygen is decreased.  So blood that is continuously moving well is keeping us warm as it’s oxygenated and unobstructed.  Think of a river thats had cleaners dumped in it (i.e. chemical shampoo/soap/detergent we use), pesticides, herbicides and hormones (from food and substances we consume), and on and on it goes. Of course the river will get clogged, lack oxygen, and you in term will be cold.  And… this isn’t just from toxic stuff we consume and in the environment. It’s also from overeating, being full all the time and not allowing the belly to empty (therefore the blood given movement)… constipation is usually a part of this story – so we’ve got to get the colon plug out, so the blood can move more freely, clean itself up – decongest, oxygenate… and viola, warm us!