Normally, blood pressure increases from resting level with exercise. Based on criteria used for exercise treadmill tests, any decrease in blood pressure during exercise might be a sign of heart disease. The same is true, to a lesser extent, if there is a failure of the blood pressure to increase with exercise. Once exercise has stopped, blood pressure should return to resting levels and may even drop to slightly below pre-exercise levels for a while based on a number of factors, like any medications one might be taking, overall fitness level, and the type and duration of exercise.
The whole question of what your fall in blood pressure really means depends on its timing. If it occurs during exercise or soon after stopping, talk to your doctor about having some sort of stress test (an exercise treadmill test or a stress echocardiogram), because, even though you feel well, this could be a warning sign of early heart disease. It may not be, but the exercise testing will help sort this out. If your blood pressure increases with exercise, then gradually decreases to a lower than pre-exercise level after something like 10 minutes or more, this may be fine
If you are still concerned, then having a stress test to be sure would not be a waste, especially given your age and the fact that you are a male. If you have any risk factors for heart disease (smoking, high blood pressure, diabetes, high blood cholesterol levels, or close relatives with heart disease before age 45), then a treadmill test might be an even better idea.
An interesting related topic has to do with “recovery time,” or the time it takes someone’s heart rate to return to resting levels after peaking during exercise. Recovery time depends on several factors, the most important of which is a person’s level of conditioning. For this reason, there doesn’t seem to be much predictive value of a certain recovery time as far as heart disease goes. There is some information, however, that specific rates at which the heart rate falls after exercise can predict increased cardiac risk, though this seems to apply only to those who are already known to have heart disease. For example, in those known to have heart disease, studies have shown that if the heart rate falls by less than 13 beats per minute (BPM) in the first minute after stopping exercise (e.g., you peak at 150 and it doesn’t get below 137 after one minute) or by less than 42 BPM after two minutes of stopping (e.g., you peak at 150 and don’t get below 108 after two minutes), there is an increased risk of dying of the underlying heart disease.