Starting About 6 Months Ago, When I Stand Quickly From

Starting about 6 months ago, When I stand quickly from the sitting or kneeling position my heart rate drops from the mid 50s to the mid 40s. This makes me feel extremely light headed, weak,, out of breath and close to passing out. When I bend over at the waist or sometimes kneel or sit down the problem goes away. I can stand back up ok but this problem soon comes back. During my 6 hour work day where I need to be up and down frequently, this happens literally hundreds of times. I am a 74 year old male. My work is very physical clearing brush and timber and operating chainsaws, bobcats and big mowers. What is you advice? Can you refer me to any medical articles that can help me understand the diagnostic procedures I need and also potential treatment options.