the same exact thing happened to me it would go slow RHR as low as 45 when lying down and upon exertion it would go too fast 120-140 but would be faint, hard to detect and I would feel very dizzy and faint, confused even.
oddly the removal of trace gluten eggs and almonds has stopped this problem. I have found that if I am exposed to a food allergen and push my self at work.. Don't "rest it off" then these symptoms will begin to return. For the most part however, this is a thing of the past for me.