FWIW, a lot of people conflate colds with the flu. If you get a bad cold, granted, you will feel miserable for a couple of days, even bad enough to stay home from work and in bed or on the couch for a day or two, but with the flu, you will be flat out on your back for a week or more.
Some of the symptoms of the cold and the flu are the same - sneezing, stuffy nose, a sore throat, a headache often due to sinus congestion, feeling a bit tired, but with the flu also comes a high fever and severe body aches, splitting headaches and very bad chest congestion and a deep and sometimes either a phlegmy or a non-productive dry cough and sometimes but not always, also nausea and diarrhea and severe fatigue, fatigue such as it is struggle to get out of bed even to go to the bathroom.
And with a cold the symptoms come on gradually, you can start with a bit of a sore or scratchy throat and sniffles a full day or two before feeling really sick but with the flu, it hits you suddenly and like a ton of bricks. With the flu you can get out of bed feeling fine, except for perhaps being a bit tired and then within hours are laid up in bed feeling like you might die and will feel that way for several days or more.
In your case I also wouldn’t discount winter allergies.
I have them and when I get a bad winter attacks, and I sometimes get an attack two or three times a year starting in late fall through mid-winter, I feel like I’ve come down with the flu. It hits me hard and comes on suddenly. I sneeze uncontrollably, watery eyes that sometimes swell up, I get chills and hot flashes (although if I take my temperature, it is usually normal or just slightly above normal) but I also get body aches and my throat will sometimes get sore and I will cough but I think it is mostly because of the post nasal dripping, but still it puts me down and I’m not able to function.
But how I know it is not either a cold nor the flu is that 24 hours later, I feel perfectly fine. It goes as quickly as it comes on.
Perhaps the change in work habits and schedule, getting more fresh air from going outside more and the supplements have helped stave off allergy attacks rather than a virus.
Wintertime allergies have several sources, among them are leaf molds and tree spores, but also and more commonly, the mold and dust that can collect in the ducts of indoor heating systems and when you start running the heat indoors, it can give you an allergy attack.
Finally someone describes the flu accurately. It kicks your ass! If it doesn’t kick your ass it’s not the flu.
Adenovirus and Rhinovirus can cause symptoms attributed as an influenza infection.