Interesting question. I just did a literature search for medical studies on the question and one thing is clear: exercise does not prevent the common cold. But does exercise reduce the duration of symptoms? By the time symptoms appear (the symptoms are the body's inflammatory reaction to the virus) you have been infected for several days. The symptoms then last from a few days to more than a week and vary within the individual and between individuals. Also, after a short time you are no longer infectious but it takes time for your body (immune reaction) to calm down. Best guess - coincidence. Your body had things well under control when you went riding.