On the quarrel between baby in the womb and suckling babies

I want to know something. It's been bothering me more now because in my current line of work, I hear and see it all the time.
When breast feeding mothers take in, their suckling babies either starts to purge, vomit milk or reject the breast milk completely. Sometimes the mothers don't even know that they are pregnant until they come to the maternity and the first thing they are asked by the nurses ọ when they complain about their babies is, 'are you pregnant?'
I found that odd, because I felt there's no need for a baby in the womb to affect the breast milk, so I asked questions but got similar responses too.
Then not just the hospital, but people close to me have experienced such too without actually knowing the cause. I and my friend were trying to conjure up reasons in our head for it, silly ones that I can't say 🤦‍♀️🤦‍♀️ and I asked a doctor who told me there's no medical explanation for that.
I went online and the English is plenty🙄, with no explicit reason for this occurrence.

So I'm here now to ask, if someone has any explanation for this?
I'm not asking for any report or something like that o, I'm just curious to know what the quarrel is between a newborn and a growing foetus. Why the beef over breast milk they (foetus) don't get to drink for months?
😒😒😒