For me, a bout with Covid in 2021 triggered severe fibromyalgia pain and insomnia. Looking back, I think I often had short flares lasting about 2 weeks when the seasons changed. At that time, I thought it was the flu. I've read it can be triggered by viruses or stress.

What helped me recover, or reset my system, was LDN - Low Dose Naltrexone. As this article says, it's believe LDN works by doing the following:
--Block proteins that worsen symptoms in the nervous system
--Act as an anti-inflammatory by reducing inflammation in the nervous system
--Balance the immune system, which could help with immune-related illnesses
--Temporarily block pain receptors and encourage the release of endorphins, the body's natural painkillers

It's safe and has no bad side effects for me, except for vivid dreams (not nightmares tho). Drawbacks are that you must get your prescription filled at a compounding pharmacy, it's not covered by insurance, and it takes a few months to work up to the maintenance dose and then can take a couple months to notice benefits. Also, I read that it doesn't benefit everyone. It costs me $115 for 90 day supply.
https://www.verywellhealth.com/low-dose-naltrexone-ldn-for-fibromyalgia-cfs-716070