oh my god I had the exact same situation as you!And we're almost the same age too,I''m 15.I was hospitalized with pneumonia 2 months ago.After i recovered,I felt different,I felt scared of everything's happening in my body.I had pain on my left arm and was constantly worrying about it.Now i have muscle spasms all over my hody and is depressed because they won't go away
I think from what you describe you're having common side effects from antibiotic treatment. Because they kill off beneficial organisms that regulate your immune system, mental balance, and digestion, some people get a very strong reaction to some of them. One way to try to fix this is to go to the best local health food store in your area and buy the best mulit-spectrum probiotic from the refrigerate section of their supplement department. This will replenish some of what was killed off. It's really unfortunate, as the flu can't be treated with antibiotics, and sinusitis isn't a disease but just means your sinuses were inflamed, probably by the flu. But that's done. Part of it probably is also anxiety and part of it might be whatever virus you had, which sounds much more virulent than common flu, might still be affecting you and frightening you.
This sounds like depersonalization/derealization. These symptoms often come with anxiety, but there is also a depersonalization disorder. It can feel really weird and scary, I know. Unfortunately, there are no medications specifically for depersonalization/derealization, but some people can benefit from therapy.