Thanks for taking the time to respond to my question. Your answer is very vague as I'm sure there are too many variables to just "guess". I received a response to this question on another forum that leaves me curious. They wondered if my arm muscles were underdeveloped compared to the level of exertion I was pushing my legs/rest of my body. This could be it. I started doing more upper body weight training, and some arm endurance training. We'll see if that helps. I'm in pretty healthy shape otherwise.
Well, without a detailed clinical evaluation it would be difficult to determine the cause of your symptoms. Possibilities that may need to be considered include overexertion, lactic acid accumulation, low blood sugar, low blood pressure, neuro-muscular/ vascular causes etc. You may consider an evaluation by a primary care physician, if a specific cause is identified, it can be managed accordingly.
Hope this is helpful.