Hi!
It's great that you are paying attention to these minute changes in your body and wondering about the causes as prevention is always better than cure.
From your symptoms, it seems that your body is under stress and present as overexertion. This looks like a sub-health condition. There are many body conditions that can contribute to feeling fatigue, the reason it gets better with rest also shows the body is in need of more ample rest time. If your body was not under physical exertion, then it could be due to rapid changes in the blood circulatory to the brain. The brain will then release signals to the body to combat the sudden decrease in blood supply, in your case, cold sweat and blurred vision. Were there any palpitations?
To rule out chronic illnesses, it is advisable to check your cardiovascular system for any abnormalities before we take preventive measures. Side note; do you have sinusitis or skin conditions from childhood? It may be a collective syndrome due to the weakening of the immune response system.
The main thing we need to work on now is to get more rest and practice certain breathing techniques for better mind-body coordination. A simple action goes a long way, the power of sound rest cannot be underestimated to deliver sufficient nutrients around the body. That being said, lean and clean diet with green leafy vegetables can improve cardiac health as well.
There is a myriad of reasons, we will need to take a more thorough look at your body condition should your symptom persist.
Get well soon!
Victoria
