Influenza is different from a cold virus and different from bacterial pneumonia, because it can cause lung tissue swelling.
Think of the lungs as having a certain amount of air space. Now, think of the walls between the air spaces getting swollen and inflamed: the air space can be cut in half. What is the result?
When the air space is cut down, in half or more, the heart has to work harder. The person may be ok when they are sitting at rest, but when they get up to walk, they cannot take a deeper breath. Their heart rate will rise to make up the difference, to try to get enough oxygen from the decreased lung space to give to the active muscles.
For example, I saw a person last week who had been sick for 5 days. No fever. Her heart rate at rest was 111. Normal is 60 to 100. Her oxygen level was fine at rest. She had also dropped 9 pounds since I had seen her last and she couldn't afford that. I sent her to the emergency room and she was admitted, with influenza A.
I have seen more people since and taken two off work. Why? Their heart rate, the number of beats in one minute, was under 100 and their oxygen level was fine. But when I had them walk up and down a short hall three times, their heart rates jumped: to 110, 120. Tachycardia. I put them off from work, to return in a week. If they rest, the lung swelling will have a chance to go down. If they return to work and activity, it's like running a marathon all day, heart rate of 120. The lungs won't heal and they are liable to get a bacterial infection or another viral infection and be hospitalized or die.
I had influenza in the early 2000s. My resting heart rate went from the 60s to 100. When I returned to clinic after a week, I felt like I was dying. I put the pulse ox on my finger. My heart rate standing was 130! I had seen my physician in the hospital that morning and he'd gotten a prescription pad and wrote: GO TO BED! He said I was too sick to work and he was right. I went home. It took two months for the swelling to go down and I worried for a while that it never would. I dropped 10 pounds the first week I was sick and it stayed down for six months.
Since the problem in influenza is tissue swelling, albuterol doesn't work. Albuterol relaxes bronchospasm, lung muscle tightness. Cough medicine doesn't work very well either: there is not fluid to cough up. The lungs are like road rash, bruised, swollen, air spaces smaller. Steroids and prednisone don't work. Antiviral flu medicine helps if you get it within the first 72 hours!
You can check your pulse at home. Count the number of beats in one minute. That is your heart rate. Then get up and walk until you are a little short of breath (or a lot) or your heart is going fast. Then count the rate again. If your heart rate is jumping 20-30 beats faster per minute or if it's over 100, you need to rest until it is better. Hopefully it will only be a week, and not two months like me!
Feel free to take this to your doctor. I was not taught this: I learned it on the job.