Welcome to HCM.
It is quite normal to have irregular periods
after delivery, especially if you are breast feeding your baby. Still to be on safer side it is better that you use some kind of contraceptive
device such as copper T, since you may not recognize your pregnancy
, mistaking it for delayed periods. You can do pregnancy test to check for pregnancy, but negative result does not exclude pregnancy but only means that the pregnancy is not of the duration which can be detected by that test. The only sure way to exclude pregnancy is when you have your normal periods.
Wish you all the best.