What causes irregular periods?

Hi, mommas, so I have a question for everyone who has ever delivered a child. So my first was an emergency c section, and this one was a VBAC. He’s now three months today; I started my period back up when he was six weeks old. A week before, I went back to work. I breastfeed and pump. So he just eats breastmilk. Well, anyway, ever since I got my period back, I’ve had it once a week!!! Is this normal, or should I call a dr??