There is a confusion of two different things here.
Bradycardia is a medical condition where an individual has a slower than normal heartbeat and it can cause the symptoms sarahale describes.
People who have done a lot of training may have adapted to have a lower than normal heartrate which isn't a problem. One of the famous cyclists (Indurain?) was said to have a resting pulse of 28. Equally though I'm sure there were other famous cyclists who never got under 50.
Those of us who have very low pulses - including me - tend to like it because we think it is something to be proud of and an indicator of fitness. The picture is much more complicated than that. It is probably about as useful as BMI, a general indicator with lots of exceptions. Mine could well be a mixture of cycling and genetics. My granny had a very low pulse, and associated low blood pressure, which did lead to fainting and culminated in her being fitted with a pacemaker. Again though everybody is different and her route very late in life wont be the same as other people's. It has lots of different causes and different treatments.
So
@sarahale diagnosis by internet is never a good idea. What causes yours is likely to be different from other people's. Read up a bit about it, be ready to ask questions, tell them your history, including whether you have trained to a high level over a prolonged period of time, and wait until the tests are concluded and the need for any management measures are agreed.