This is just a thought, but when you are training your heart rate is probably at above 65% of your max heart rate. If its a hard ride maybe even above 80%. So say your max is 200 as an example, your 65% is 130 and your 80% is 160, when you get off the bike your body isn't demanding oxygen and nutrients from the blood as quickly so your heart rate will probably drop to under 50% (100bpm), a drop of at least 30bpm (probably far more). The blood flowing through your body helps to keep you warm and maintain your body temperature, but now its not doing that as fast and you start to get cold. Strip off any sweaty wet clothes quickly, eg socks, tights worn over knicks, armwarmers and quickly replace with a warm fleecy tracksuit if you can. If you can duck into a rest room for a minute its probably worth taking off all your clothes, towelling dry and putting warm stuff on.