Periods could get out of whack at any time. Stress can especially do this. Hormones being out of whack is another, You could have had a chemical pregnancy. That will put your periods out of whack. I've never heard of a flu that interferes with periods. And if you don't ovulate on your own well nothing is impossible so you could get pregnant at any time but that isn't likely. Right now you may think you are getting it but you aren't. Could be implantation bleeding.
I mean the possibilites are endless. If you are curious if your pregnant take a test if it's negitive and still no period a week later take another again if you feel like you are. But a out of whack period can happen at any time. I'd let it go for a few months. It can just happen before I even bother with the gyno. But I'd take pregnancy tests regularly.