I definitely think you should talk to your doctor about this.
Your wonky periods could be due to almost anything.
Please don't believe the myth that you can't get pregnant while breastfeeding. My uncle and dad were born so close together that they were raised as twins. I have a friend who had 3 kids born exactly one year apart. Right now, they are 4, 5, and 6 years old. They keep her so busy that her head could spin off most days.
If you're not ready for another baby, talk about birth control options.
I know women who took the depo shots and were very happy, but if it's not for you, discuss an alternative so you don't have to go through the constant worry.
Best wishes.