Hi, I'm An Active 64 Year Old Male. About Once

Hi,
I'm an active 64 year old male. About once a month after doing some heavy gardening with lots of bending over, or sometimes after a vigorous workout, I'll suddenly start feeling very weak in my limbs, with leg, arm and hand tremors. I'll really start sweating and feeling light headed. It's like I've just run a 10k at top speed, except I'm not breathing that hard. I usually have to stop and sit down and it'll take 30 to 45 minutes for me to start feeling normal again. My heart's good and my blood pressure doesn't go up or down during these episodes. I've discussed this with my primary care physician, but we haven't been able to come up with anything. Am I just getting older? Thanks, any thoughts are appreciated.