My bloodpressure reading directly after strenuous exercise was 138/118? what could cause the diastolic pressure to rise so drastically.

High diastolic: If the high diastolic pressure quickly normalizes after exercise, I would tend to regard the very disproportionately high diastolic reading as inaccurate. Studies using very accurate intraarterial blood pressure monitoring reveal that our external blood pressure monitors usually over estimate diastolic readings and underestimate systolic readings. Nevertheless continue frequent checking of BP