I'd second Gently's suggestion to contact your physician ( ?cardiologist) about your low heart rate, especially if it's continuing and you're feeling bad as a result of this heart rate. A heart rate monitor ( either a Holter or one of the longer monitoring monitors) should give you, and your doctor a good picture of what is happening with your heart.
Your heart rate may well be a lower rate ( normal for you) as a result of your athletic activity, but how low is normal for you? If you're symptomatic with those lower heart rates it seems to me that could be a problem that needs to be checked out.

And congratulations on your achieving the NED status with your cancer. Best wishes that it will stay that way for a long, long time!