Heart pounding/Headache

[Fpage1]

I consider my self in great shape. I am 59 years old. I work out out four days a week. I begin my 1 hr routine at 4;30am. about three hrs after working out , after sitting at my desk, whenever I stand up my heart pounds and i feel as though my head is going to pop! Then after 30-45 seconds it passes. if I sit again and then stand, the same will occur til the late afternoon. On days I do not work out, this does not happen. I have ask my Dr. and he has no explanation. Any one have any ideas?