I’m sure it’s nothing:

The “but you should still get one part” is reflexive at this point. It’s how they’re are able to get away with reporting bad things about the Vaccine™ without getting canceled.
They could publish a study that shows there’s a 75% chance your spontaneously bursting into flames on the spot, and it would still be headlined with “New study shows 75% chance of immediately bursting into flames upon receiving Covid-2019* vaccination, but experts say vaccines are still safe and effective.”
It’s required boilerplate, essentially.
So now we do have at least some evidence that that Covid-2019 shots can have an effect on peoples’ reproductive systems.
As I am a man who has never obviously had a period, I’m a little out of my depth here so really the above sentence is all I can draw from this: that the vaccines have displayed the ability to affect the reproductive system.
I did a bit of googling on things that can potentially mess with a woman’s period, and this was what I found on health.com. I thought it was at least interesting and noteworthy:
Any medication that involves hormones—like thyroid medication (and thyroid problems in general), steroids, or antipsychotics (which release a hormone, dopamine)—can influence your period. The first thing you should know is that hormones don’t act in isolation. “All hormones circulate throughout your bloodstream, so they’re all connected, even if they’re produced in different glands,” says Dr. Puritz. Plus, some hormone receptors look a lot like other hormone receptors, so a drug can easily mistake its target hormone, affecting a fertility hormone as well as the intended thyroid hormone, for example.
As far as I know, the Covid-2019 shots don’t involve hormones, do they? Again, I’m way out of my depth here, but this article makes it seem as if the main way a medication can mess with a woman’s period cycle is by altering hormone levels. This is not enough evidence to conclude that the Covid-2019 shots alter your hormones, but it is at least a possible explanation.
Here’s what the NPR article has to say:
A new scientific study shows that vaccination can cause changes to the timing of menstruation. But it also shows the effects are temporary, more akin to a sore arm than a serious adverse event.
“I think it’s reassuring and also validating,” says Dr. Alison Edelman, a professor of obstetrics and gynecology at Oregon Health and Science University in Portland, Ore. who led the study.
The work appeared Thursday in the journal Obstetrics and Gynecology. Edelman and other experts stress that individuals should get vaccinated, because the risks from COVID-19 remain high.
No, they don’t. The risk from Covid-2019 have never been “high,” and they are not “high” currently. This is flat out false.
This is a completely ridiculous way to even talk about it. Covid-2019 does not affect everyone the same. Your risks with Covid-2019 are heavily dependent on your age and your underlying health conditions. Some people have elevated risk for Covid-2019, but the vast majority of people have very little risk at all.
It is astoundingly ignorant and irresponsible to say “the risk from Covid-2019 remain high” for every single person in the country.
many people did notice changes to their menstrual cycles. A survey conducted by anthropologists found numerous reports of unusually heavy flows and even breakthrough bleeding among some people who hadn’t menstruated in years.
Anti-vaccine activists capitalized on other anecdotal reports from social media–using them to make unfounded claims that the vaccines were being used to spread infertility and ultimately depopulate the earth.
Okay, but when women were posting on Facebook that their period cycles were messed up after taking the vaccine, FACT CHECKERS immediately DEBUNKED those claims:

So much for that, huh?
Now there’s still no evidence that the shots affect fertility, and I certainly hope to God they don’t, but we can see how quickly new information comes out and makes people who were making definitive and authoritative pronouncements look stupid.
Anyway, let’s get on with the article:
Researchers still aren’t exactly sure why the changes occur. Edelman says the immune system and the reproductive system are linked, and some signaling immune cells also can affect the body’s natural clock. Given that relationship, it’s entirely possible that triggering the immune system through vaccination might change a person’s cycles. But whatever’s happening seems to be short-lived.
It’s good that it appears to be short-lived, but it’s also a bit concerning that researchers still have no idea why the Covid-2019 shots are altering women’s periods. And it’s even more concerning that they’re saying, “We have no idea why this is happening, but it’s probably nothing so get your shots anyway.”
Every concern about the shots is brushed aside and dismissed.
Like I said earlier, a study could show an 80% chance of your dick falling off after receiving a Covid-2019 shot and they’d still say “vaccines are safe and effective.”
And I love how the media just slipped this out there on January 6, the day of the whole coordinated and contrived media sobfest over The Day Our Democracy Nearly Died.
I’m sure that’s just a coincidence.
Vaccines are safe and effective, and don’t you forget it.
* I am now calling it “Covid-2019” instead of “Covid-19.” I want it to be perfectly clear when this virus was introduced to the world, and how long our lives have been upended by it.