A woollen plunderhouse codpiece is the way ahead, preferably in merino wool.
http://www.lynnmcmasters.com/sturecodpiece.html
I think Stef F is right - this is not a cold related problem - if it was it would not happen in a gym - but related to distortion/pressure on the relevant body parts.
I suspect this problem is a form of Raynaud's phenomenon, where blood vessels go into spasm and circulation is shut off in over-reaction to cold, but also to stress, either physical or mental, and sometimes other triggers eg caffeine. The body panics and seeks to preserve it's core by sacrificing what it sees as less important body parts. These are more usually fingers and toes, but can also be noses and cheeks. Clearly breasts are not necessary for survival and are therefore appropriate as sacrifices.
Strategies for coping include keeping the core warm, keeping the affected parts warm, and anything that may improve circulation from Nifedipine to Viagra. I have recently detected some benefit from daily beetroot juice, but it certainly hasn't solved the problem. I once felt so fed up with the condition I got some Viagra from my GP, but reading the list of likely long-term side effects was enough to deter me from taking any.
Cyclops, do your breasts feel cold to the touch when you are having problems, and maybe change colour, going white, red or blue? - if so, I think my Raynauds diagnosis is correct. Raynauds is usually a stand alone condition (primary Raynauds), quite common in young women, but it can also be a symptom of other maladies (secondary Raynauds). Your GP should refer you to a rheumatology department, where they would do lots of tests to make sure which you have. This is important, as Raynauds can be a symptom of various immune system problems which need early treatment.
thanks Alexandra for the helpful information, and to other useful stuff. I'll monitor the situation for a while as my observations so far have been irratic, usually just noticing when it's so painful I can't think of anything else. I'm familiar with raynaulds thoough more for hands so will research this more. thank you. I'm also thinking the tightness of the bra around my chest/back may be an issue so will look to get another bra and see how that goes.
Hi cyclops - this solution depends on how well-off you are in the breast dept. I'm built like a lamppost and therefore wear those super-enhancing Wonder-where-they've-gone bra's with not so much a chicken fillet insert as the whole damn bird.. One rather lovely upshot of this gel insert is that I now have terrifically warm breasts and have on occaision whipped out the insert and used them as impromtu hand-warmers. They seem to store heat fabulously and whereas I've suffered in the past with cold boobies, nowadays I get to look hot and feel warm too!!#
Just a thought. Obviously if you're built like a normal woman then you'd probably not be wearing this kind of brassiere on your runs...