As I read it:
I had a flu, I applied to reschedule and after 3 days, they still did not get back.
Well, nothing to panic, yet.
Usually, it is estimated that, for a simple flu it takes 5-7 days for the symptoms to go away and 7-10 days to recover completely. maybe the company just wants to reschedule on a later date, giving enough room for you to recover complete (if the interview is remote) and to avoid possibility of getting other employees infected (in case in-person interview). They just may not have a fixed schedule for the interview that far ahead (2-4 weeks) and taking their time to fix a later date on which it would be possible for them to get that interview arranged.
Get in touch with them, (over phone, if you know the contact numbers, if you don't, search/ask for it) and gently remind them about the requested reschedule. As mentioned in other answers, emails tend to get lost, never fully rely on them.
However, now it is too early and not correct to think about
Is this a red flag? Should I re-apply? Is this a weird rejection?
You don't have any grounds to start thinking about these.