Hello,
No you should not ignore this. See a doctor as soon as possible. It is not likely to do with the cortisone shot but should not be ignored.
Regards

I'm sorry about your complaints, no this is not normal, and may or may not be related to the cortisone shot. Do you have any other symptoms? Cortisone can temporarily weaken your immune system, allowing opportunistic pathogens, like viruses, to re-emerge. It is unlikely however, this is the case, but rather a coincidence that you had the shot several days ago and subsequently fell ill. It is prime flu season, and muscle aches are common with flu and common colds, perhaps coincidental. If you have a history of osteoarthritis, a cold can cause a flare up. Alternatively, if this is related to the cortisone injection, we would be looking at an inoculation event, and you would have bacteria in your blood from the injection, be very sick, and possible have seeded your left shoulder with this bacteria. A very unlikely event, even less so if your injection site is fine.

Rest up, hydrate, take Ibuprofen for your shoulder pain, Tylenol for your fevers, and hopefully it will pass in a couple days. Keep me updated.

Marco Zahedi, MD
Family Medicine