No, do not worry about it. You took those pills for quite some time.
And if you want, I don't see why you can't start the prenatal vitamins already. Or, if those are more expensive than regular ones, just take a good multi vitamin and taken an extra folic acid, which is vital for fetal development.
Good luck
There is no need to worry and yes, your guess is correct it will take some time to have regular periods. And this cause is because some of the pregnancy hormones are present in the body till then you'll have irregular periods.
And start taking prenatal vitamins before you know that you are pregnant.