I've had irregular heartbeat/palpitations for years but this feels different. brief episodes of intense rhythmic pain in my heart. what could this be?

Need a monitor: Your symptoms are most likely related to an irregularity in the heart (cardiac) rhythm. The kind of symptom complex you describe may be associated with a benign cardiac rhythm disturbance, like single premature beats, but it may also be associated with a more significant arrhythmia. A 24 hour cardiac monitor would be helpful in determining the cause of your spells.