What could cause an overly strong heartbeat?sometimes walking up one flight of stairs, hr will go up to 150.i'm in great shape.uncomfortable pounding

Many reasons: If your heart rate increases a lot with minimal activity, it could be a sign of low blood pressure from anemia (like with heavy cycles), dehydration (not drinking enough water), or hormone imbalances (diabetes, thyroid, adrenal or female). Could be stress too. You should see a primary care physician for exam and blood tests. If all normal, then see cardiologist for EKG, Echo, and holter monitor.