My SO hasn't had a regular cycle after taking the first vaccine shot (Pfizer) way back when it became available. It's slightly better now, but she used to have

My SO hasn't had a regular cycle after taking the first vaccine shot (Pfizer) way back when it became available. It's slightly better now, but she used to have very regular cycles before. She asked her GP and gynecologist about it and they both served some waffle about "we don't know why but nothing to worry about". Like how can you say it's nothing if you don't know the cause...