Yeah this doesn't seem to be an after-riding thing, it seems psychosomatic or nerve based or something. The heart will launch into a vastly-accelerated heartrate for any number of reasons, including hormonal stress (i.e. bad dreams or even no emotion but some brain chemistry imbalance triggering an adrenal release or a CNS message that raises heart rate), elevated/depressed breathing (demand for oxygen raises heart rate to get more blood to the lungs, too much oxygen causes a feedback loop), or a small infarction of the coronary arteries (i.e. a miniature heart attack; heartrate goes up when the heart needs oxygen, so when your body over-consumes oxygen or when the heart fails to get oxygen to itself it will speed up).

That's all well and good, but the short version is "it could be anything." A heart attack, a bad dream, sleep apnea (not breathing while asleep), stray nerve impulses, bad air circulation reducing the amount of oxygen in my room, etc.

This is why doctors tell you to not self-diagnose huh :/