Hello, I can understand your concern. It is normal for the heart beat to change with exertion. It is slow when resting and becomes more rapid with fast movements of the body. Blood clot in a thigh vein can dislodge and go to the lung circulation ( pulmonary embolism). A person can have a fast heart rate and breathlessness with this problem but it will be always there and will not slow down with rest. It is a serious condition which is a medical emergency. You would not be able to do stress test with a pulmonary embolism. So relax, there would be another cause and if the stress test was normal, there is nothing to worry. I am ready to answer you if you have further queries. Dr. Noble Zachariah, Kuwait. If this answer has helped you, please indicate it , to guide others with similar queries.